Sure, but we're not talking about cultures, but about species. Vulcans and Romulans belong to different cultural groups but not to different species, at least if we're not using the most common definition
of a species as "a group of organisms capable of interbreeding and producing fertile offspring." Vulcans and Romulans are interfertile, as one would expect given that they've the same homeworld and were separated only two millennia ago, hence they belong to the same species.
Well, by that definition, pretty much all Trek humanoids are the same species.
Still, even by Trek definitions, you're right that Vulcans and Romulans are the same species. The populations only diverged 2000 years ago, and even by Trek's wonky genetics, that's not enough time for speciation.