IIRC the cause for the split isnt completely fleshed out, but supposedly it happened around that time with the romulans rejecting the new philosophy. I am not sure if they simply left or were exiled though.

What I also find interesting in this regard is that the vulcans are slightly telepathic, and the romulans are not. Perhaps it is an ability that developed from generations of mental discipline the vulcans went through.

Another fun (and not very likely) take I once read is that the vulcans we know are actually augments, similar to the ones earth made and banned after they became tyrannical.

The theory here being the original vulcans (physically romulans) tinkered with genetic engineering, creating the modern form of the vulcans with increased mental capacity and psionic potential, also losing the romulan forehead in the process. Then they would go the same route as the human augments, paired with their violent emotions, and start a civil war with their regular brethren who then eventually lose this war and leave or are exiled.

The new vulcans then start warring with each other until eventually surak shows up with their modern philosophy.