The Rommies may make some BS claim that they have a legal right to the planet due to their history but no one would buy it and it would be time for two super powers to go to war.
Yet if Vulcan decides it wants
to get conquered, one wonders if Starfleet or the UFP would really do anything about it. Vulcan is supposedly a worthless rock in terms of resources, and its strategic position at the apparent threshold of Romulan space would not change much for the worse even if it ended up being an enemy world. If the Federation did not respect Vulcan right for self-determination, and took the planet back by force, would this benefit or hurt the Federation?
The idea of Vulcans seceding from the UFP at the earliest excuse is not all that unlikely, considering the history of the planet and its alliances. A Vulcan-Romulan(-Reman) alliance might well be preferable to clinging on to the silly humans and their comrades-in-ignorance, and an "invasion" by a small force of fanatics might well be a welcome turn of events. Except perhaps for the factions represented by Spock on the two worlds, but then again, Spock was always in a distinct minority with his thinking.