Vulcans and Romulans probably diverged more recently than some races of humans on earth, say a few thousand years ago
According to TOS's "All Our Yesterdays," Spock's ancestors a mere 5,000 years ago were savages. That would be some awfully fast history, with starflight, a possible dark age and everything else thrown in. (Remember that Vulcan logic is a philosophy, not a behavioral trait. Yet Spock's trip through the atavichron is enough to strip that away, as though it were genetic.)
Is is possible that Vulcan
is the colony and Romulus the mother planet? Or perhaps both are colonies of a third planet?