From The Enterprise Incident
"There's only one Vulcan aboard that ship.
He should be easy enough to locate.
Romulans and Vulcans read almost exactly alike.
There is just a slight difference which --
Got him, sir."
"Almost exactly alike" isn't the same as "exactly alike". There are differences otherwise Spock would still be on the Romulan ship. And in genetics, almost compatible isn't the same as compatible. Human and chimpanzee DNA is almost the same but that doesn't mean you can create a human/chimp hybrid.
Granted that there are differences between the Romulan and Vulcan populations on the whole--I've granted that already--I have to wonder how the difference between Romulans and Vulcans could be significant to the point that they belong to different genuses.
Neanderthals and human developed many significant differences, brain capacity in the former species being 1600 cubic centimetres versus early
homo sapiens sapiens' 1300 cubic centimetres, this difference having evolved after tens of thousands of years of isolation in separate environments, but it turns out that the two populations were still pretty darn interfertile when they were brought together. How could Vulcans and Romulans--never mind Vulcans and the other Vulcanoid civilizations out there--have plausibly diverged so much in such a short period of time?
Regarding Simon Tarses and Spock, they are human crossed with Vulcan and Roumlan respectivly, not Vulcan crossed with Romulan. Still not proof of Vulcans and Romulans being cross fertile.
Not directly, no. It is proof that there's enough overlap between the Vulcan and the Romulan gene pools for an individual with ancestry in the second demographic to claim ancestry in the first. If there's that much overlap, then assuming interfertility isn't possible would beg the question of why individuals from two populations with such significant genetc similarities can't reproduce.
There wasn't a lot left on the transporter pad when T'Pel was beamed off but that doesn't matter since whatever it was, it wasn't her.
The genetic material that the Romulans transferred to the
Enterprise-D was replicated, but it was otherwise identical to her own DNA--it had to be, else someone less competent than Data and Crusher could have picked it up.
Arne Darvin was an assistant to the Federation Undersecretary for Agriculture and yet he was a Klingon, a fact McCoy confiremed with a tricorder and about 5 seconds of time. It would seem that the Federation isn't in the habit of scanning their civilian representitives so it's not surprising that T'Pel slipped through as well.
But when the replicated DNA was analysed in detail by Data and Crusher, they determined that the DNA was replicated. They did not determine that the owner of the original DNA was Romulan--they found that out only when the former T'Pel appeared on ther viewscreen dressed in the garb of a Romulan subcommander.
NuSpock's line stating that only ten thousand Vulcans remain could, if the writers of the new movies decide, be a definitive statement, with the Romulans and other Vulcanoids being slotted off into different species somehow and there being no significant offworld Vulcan populations at all. They've the right to determine the canon.
That intepretation isn't necessary, however. The line lends itself to multiple interpretations, the most restrictive of which has been excluded by the writers of that line, and by explicit and implicit statements within the TV series about the relationship between Vulcans and kindred populations never mind the novels and other ancillary materials. Again, if the NuTrek writers--or other writers charged with writing canon, if there are ever future TV series, say--explicitly state that Vulcans and Romulans are separate species incapable of reproducing with each other without radical technological interventions, then those statements would be compromised. Given what's been established already, that would seem unlikely.