The earliest period in Vulcan prehistory we have any information about was many thousands of years before the split between Vulcans and Romulans.
In "Return to Tomorrow" the Enterprise encounters a voice from a long dead planet:
SPOCK: With two very important exceptions. It's much older than Earth, and about a half million years ago, its atmosphere was totally ripped away by some sort of cataclysm. The planet has evidently been dead since then. Sensors detect no life of any kind.
Later:
KIRK: That's twice you've referred to us as my children.
SARGON: Because it is possible you are our descendants, Captain Kirk. Six thousand centuries ago, our vessels were colonising this galaxy, just as your own starships have now begun to explore that vastness. As you now leave your own seed on distant planets, so we left our seed behind us. Perhaps your own legends of an Adam and an Eve were two of our travellers
MULHALL: Our beliefs and our studies indicate that life on our planet, Earth, evolved independently.
SPOCK: That would tend, however, to explain certain elements of Vulcan prehistory.
http://www.chakoteya.net/StarTrek/51.htm
But that was probably hundreds of thousands of years before the Romulans split from the Vulcans.
MY theory is that the Vulcans developed interstellar faster than Light travel, possibly with warp drive, many thousands of years ago and established many Vulcan colonies on planets of other stars, often far distant from Vulcan. And the Vulcans were violent and aggressive during this space colonizing era.
In "Balance of terror" Spock says:
SPOCK: Yes, indeed we do, Mister Stiles. And if Romulans are an offshoot of my Vulcan blood, and I think this likely, then attack becomes even more imperative.
MCCOY: War is never imperative, Mister Spock.
SPOCK: It is for them, Doctor. Vulcan, like Earth, had its aggressive colonising period. Savage, even by Earth standards. And if Romulans retain this martial philosophy, then weakness is something we dare not show.
http://www.chakoteya.net/StarTrek/9.htm
So Spock thinks that Romulans are probably a lost Vulcan colony from that time.
I think that some disaster destroyed the Vulcan space travelling civilization, maybe a foreign or civil war. Civilization on Vulcan and its colonies collapsed, and artifacts of that civilization were gradually destroyed on the planets, while wrecked space stations and space ships remained in outer space to be discovered thousands of years later.
And apparently all records of the locations of various Vulcan colonies were lost, so that when the location of the Romulan home world became known, nobody realized that there had once been a Vulcan colony in that system.
In "The Paradise Syndrome" Spock says:
SPOCK: You prescribed rest, Doctor. The symbols on the obelisk are not words. They are musical notes.
MCCOY: Musical notes? You mean it's nothing but a song?
SPOCK: In a way, yes. Other cultures, among them certain Vulcan offshoots, use musical notes as words. The tones correspond roughly to an alphabet.
http://www.chakoteya.net/StarTrek/58.htm
Spock's words imply that there are at least two Vulcan offshoot cultures, presumably descended from lost Vulcan colonies, which use musical notes as words, and that there are other Vulcan offshoot societies which do not.
It is possible that the Vulcans died out on most of the lost Vulcan colony worlds, so that the surviving Vulcan descended societies are not as numerous as the human-looking aliens.
And after thousands of years civilization was rebuilt on Vulcan and Vulcans rediscovered interstellar travel and created some more interstellar outposts and colonies, including the spiritual retreat of P'Jem on an isolated planet.
In the Enterprise episode "The Andorian Incident" they visit P'Jem:
TUCKER: You say this is a place to purge emotions? Looks like somebody had to purge pretty bad. He bashed the door in.
T'POL: The temple is almost three thousand years old, Commander. You can't expect it to be in pristine condition.
So about AD 2150 minus about 2,500 to 3,000 years means the temple at P'Jem was founded about 850 BC to 350 BC, and so Vulcan rediscovered interstellar travel sometime in or before that date range.
Sometime after that wars almost destroyed civilization on Vulcan again.
In "The Savage Curtain an image of Surak, who taught the Vulcans the philosophy of logic and emotional control says:
SURAK: In my time on Vulcan, we also faced these same alternatives. We'd suffered devastating wars which nearly destroyed our planet. Another was about to begin. We were torn. But out of our suffering some of us found the discipline to act. We sent emissaries to our opponents to propose peace. The first were killed, but others followed. Ultimately we achieved peace, which has lasted since then.
http://www.chakoteya.net/StarTrek/77.htm
But the wars before the peace had devastated Vulcan, and appaently the Vulcans concentrated on rebuilting their planet for many centuries before returning to interstellar travel.
In the Enteprise epsiode "The Forge":
FORREST: Ambassador. Are Vulcans afraid of humans? Why?
SOVAL: Because there is one species you remind us of.
FORREST: Vulcans.
SOVAL: We had our wars, Admiral, just as humans did. Our planet was devastated, our civilization nearly destroyed. Logic saved us. But it took almost fifteen hundred years for us to rebuild our world and travel to the stars. You humans did the same in less than a century. There are those on the High Command who wonder what humans would achieve in the century to come, and they don't like the answer.
So about AD 2150 minus 1450 to 1500 years means the devastating wars and the Reforms of Surak happened before about AD 650 to 700. How much before depends on how long Vulcan had space flight before the era of Enterprise.
Vulcans had interstellar travel before first contact with Earth in Star Trek: First Contact circa 2063, putting the era of the wars and the Reforms of Surak back to earlier than about 563 to 613.
In the Enterprise episode "Carbon Creek" T'Pol tells a story about her mother's mother's mother spending some time on Earth:
T'POL [OC]: They'd gone to Earth to investigate the launch of its first artificial satellite, called Sputnik
So Vulcan was already monitoring Earth with spy satellites and/or long range probes by AD 1957, which puts the devastating wars and the Reforms of Surak by about 457 to 507.
Some fans think that there is a clue to date of Surak in "Amok Time":
SPOCK: This is the land of my family. It has been held by us for more than two thousand Earth years. This is our place of Koon-ut-kal-if-fee,
So Spock's family has owned that land for about 2,000 to 3,000 years before about 2267 or since about 733 BC to AD 267.
McCoy speculates:
MCCOY: And they still go mad at this time. Perhaps the price they pay for having no emotions the rest of the time.
And if someone accepts the validity of McCoy's speculation, they might believe that the Reforms of Surak must have happened by AD 267, and possibly before 733 BC. But I think that the Vulcan mating drive is too biologically strong to be any sort of effect of the Reforms of Surak.
In "The Forge":
T'POL: Actually, they're a small group of Vulcans. They follow a corrupted form of Surak's teachings.
ARCHER: Surak I've heard of. He's the father of Vulcan logic.
V'LAS: Even after eighteen hundred years, we consider him the most important Vulcan who ever lived.
http://www.chakoteya.net/Enterprise/83.htm
So about 2154 minus about 1,750 to 1,850 years puts the era of Surak at about AD 304 to 404, and thus the Vulcans returned to the stars 1,450 to 1,500 years later about AD 1754 to 1904.
In the TNG episode "Gambit Part 1"
RIKER: What else do we know about Barradas Three?
DATA: The planet was used as an outpost for the Debrune approximately two thousand years ago. The Federation's Archaeological survey has catalogued numerous ruins on the surface.
So the Debrune were advanced and numerous enough to have interstellar travel and outposts or colonies on various planets of different stars about 1,950 to 2,050 years before about 2370, and so about AD 320 to 420, almost totally overlapping with the era of Surak.
Later in "Gambit Part 1":
DATA: Perhaps these artefacts have a special value to the Romulans
WORF: The Romulans?
DATA: These structures were built by the Debrune. That race is an ancient offshoot of the Romulans. The ruins on the planet where Captain Picard was killed were also Romulan in origin.
So the Romulans, on a hypothetical lost Vulcan colony world, must have rebuilt their civilization and regained interstellar travel sometime before the Debrune split off from them, which must have been sometime before the Debrune colonized Barradas Three about AD 320 to 420. Thus the evidence indicates the Romulans have had interstellar travel before the era of Surak and and can not be anti Surak exiles from Vulcan, as is popularly supposed by many fans.
Romulan history must have had some significant setbacks after rediscovering interstellar travel thousands of years before the eras of Star Trek shows, since they have not conquered and colonized Vulcan, Earth, the Gorn, the Klingons, etc., and since there are Romulan ruins on planets not ruled by the Romulan Empire in the era of TNG.
And that is my theory.
Added 12-29-2202.
An alternate Theory, which I thought of today, is that Vulcan didn't have interstellar travel before a few thousand years ago, but Spock knows that Vulcans left Vulcan many thousands of years before Vulcan had space travel. Presumably advanced aliens visited Vulcan and took some Vulcans from the planet, leaving evidence of their visits behind in archaeological deposits and/or Vulcan traditions, explaining the Vulcan-like people found on many worlds.
Those aliens might have taken Vulcans from Vulcan to other worlds for various purposes, such as being food animals, slaves, pets, oppressed workers, equal citizens, mercenary warriors, etc. Or they may have settled those Vulcans on planets and left them alone to see what would happen.
The dialog in "The Paradise Syndrome" quoted above continues:
SPOCK: You prescribed rest, Doctor. The symbols on the obelisk are not words. They are musical notes.
MCCOY: Musical notes? You mean it's nothing but a song?
SPOCK: In a way, yes. Other cultures, among them certain Vulcan offshoots, use musical notes as words. The tones correspond roughly to an alphabet.
MCCOY: Were you able to make sense our of the symbols?
SPOCK: Yes. The obelisk is a marker, just as I thought. It was left by a super-race known as the Preservers. They passed through the galaxy rescuing primitive cultures which were in danger of extinction and seeding them, so to speak, where they could live and grow.
MCCOY: I've always wondered why there were so many humanoids scattered through the galaxy.
SPOCK: So have I. Apparently the Preservers account for a number of them.
And maybe the Preservers also account for some or all of the Vulcan like species in the galaxy.
So maybe most of the Romulans are descended from a lost Vulcan colony as in my first theory, or descended from primitive Vulcans brought to another world by advanced aliens, and maybe some ancient Romans were brought to the Romulan homeworld by the Preservers and had some influence on Romulan culture. And possibly even some refugees from the Reforms of Surak stole the last remaining interstellar ships left on Vulcan, like Khan, and fled to the stars, and eventually reached the Romulan home world and introduced another element into their culture, or maybe were brought there by the Preservers.
I guess that the fan theory that the Romulans are descended from anti-Surak refugees, which can't be entirely true for chronological reasons, can be partially true if those refugees from Vulcan found the Romulan home world already settled by Romulans and joined them.
