In the days of Captain Archer, before there was a Federation of Planets, if the Klingons decided to launch an all out assault on Earth, would the Vulcans rally to our defense?
No, Earth had no mutual defence pact with Vulcan in the era preceding the creation of the Coalition of Planets and the UFP. This is most closely evidenced in the days immediately following the Xindi incident in 2153.
Acting on false intelligence from the Sphere Builders, the Xindi launched an attack on Earth with a weapon of mass destruction. Earth requested assistance from the Vulcan High Command and were (evidently) rebuffed. In addition to refusing to send any ships to aid Earth they recalled their sole officer serving aboard a Federation Starship. When she refused, she was stripped her of her rank and dishonourably discharged.
T'POL: You thought it was crucial to place a Vulcan on Enterprise during its first mission. Why not now?
SOVAL: You were there to provide logic to a crew of humans who insisted on leaving before they were ready, but logic can't help them inside the Delphic Expanse.
T'POL: Can you be certain of that?
SOVAL: The High Command was quite specific. You're to return to Vulcan.
T'POL: I believe that should be my decision.
SOVAL: This is not the matter of choice. Defying the High Command would mean immediate dismissal. You know that.
Moving further down the canon scale, the Enterprise novel : Kobiyashi Maru deals extensively with the (political) aftermath of the events of the Enterprise TV series, detailing the terms of the Coalition of Planets in 2155. Membership to this organisation was apparently predicated on a mutual-defense pact between United Earth, the Confederacy of Vulcan, the Andorian Empire and the United Planets of Tellar.
At this point, an attack by the Klingon Empire would have resulted in Vulcan ships defending Earth and vice versa.