The Romulans separated from the Vulcans during the Time of Awakening (4th century AD, Terran Calendar).

They are still the same, biologically, and can interbreed. This means nothing in the Star Trek universe, as pretty much anything can breed with anything.

Romulans have several differences from Vulcans. They are subtle, but enough to prevent Crusher from treating Patahk successfully with a technique that works on Vulcans. In that episode it was discovered that Romulans actually have a high degree of compatability with Klingons, enough that a Klingon treatment worked for the Romulans where the Vulcan treatment failed.

The Romulans separated from the Vulcans during the Time of Awakening (4th century AD, Terran Calendar).

They are still the same, biologically, and can interbreed. This means nothing in the Star Trek universe, as pretty much anything can breed with anything.

Romulans have several differences from Vulcans. They are subtle, but enough to prevent Crusher from treating Patahk successfully with a technique that works on Vulcans. In that episode it was discovered that Romulans actually have a high degree of compatability with Klingons, enough that a Klingon treatment worked for the Romulans where the Vulcan treatment failed.

Edit: To answer your other question, they are likely more closely related than humans and chimps, though this can't be guaranteed (radically different environments are one of the biggest causes for rapid evolutionary change, though this may be mitigated by a heavily technological society).