In "The Chase" we learn that many of the Alpha Quadrant species are related to a proto-humanoid species that seeded various planets with genetic material. These species included the Romulans, Klingons, Humans, Kardassians and we can assume others (Vulcans). It would stand to reason the genetic similarities amongst these species would be greater than species from outside of this population. Therefore, romulans, would share a greater family resemblance with vulcans, but it would not be overly bizarre to have crossbreeding between any of these species. Finally, in regards to the highly illogical comment by @mojoblowjo. You have conflated social categories with biological categories. Species cannot be 'more' evolved, that's not how evolution works. Biological fitness is determined by the environment and since these species control their environment to a higher degree your assertion of a 'more' or 'less' evolved species is divorced from science and therefore highly emotional. Be careful with that view, it is what undergirds all forms of scientific racism.

In "The Chase" we learn that many of the Alpha Quadrant species are related to a proto-humanoid species that seeded various planets with genetic material. These species included the Romulans, Klingons, Humans, Kardassians and we can assume others (Vulcans). It would stand to reason the genetic similarities amongst these species would be greater than species from outside of this population. Therefore, Romulans would share a greater family resemblance with Vulcans, but it would not be overly bizarre to have crossbreeding between any of these species.