Actually, we DID have at least one story of a Romulan spy posing as a Vulcan in TNG.
Then I must have forgotten it, because they didn't use it a lot. But we also had not just Vulcans but humans and androids posing as Romulans, and all sorts of other combinations (humans posing as Klingons in DS9, Cardassian posing as Bajoran and vice versa, etc.) through the extremely convient surgical alterations, which seem like the easiest thing ever in Trek.
I guess nobody ever gives those people medical checkups or thinks of checking their DNA. But speaking of DNA - shouldn't Romulans and Vulcans be the same species? There has been so much inconsistency on that matter in ST. There's even a scene in DS9 in which someone asks Bashir "aren't Romulans identical to Vulcans?" (I guess they thought the forehead ridges were just some kind of hat, eh?
) and Bashir answers: "Yes, but there are genetical differences"?!