It's never stated outright how long ago Vulcans diverged genetically from Romulans.
It's interesting to say they are the same species. Technically most of the races in the galaxy are the same species, because the scientific definition of a species is a group of organisms who can produce fertile offspring. So by the scientific definition, humans and klingons are the same species, as are most of the humanoid races in the galaxy.
Vulcans and Romulans probably diverged more recently than some races of humans on earth, say a few thousand years ago, so they probably are the same species even though they're always referred to as different species.
And I suppose if Simon Tarses could fool all the blood tests, that confirms that they are biologically indistinguishable.