I got to thinking this morning as I was driving in to work...
* After the destruction of Vulcan, Spock estimated there were only 10,000 Vulcans left.
* Due to their pon farr cycles, Vulcans would have a slow rate of reproduction.
* The Romulans of that time period were in favor of reunification by force.
* Assuming the invasion was put off until the time Sela appeared in the mid 2300's as in TNG, there's probably no more than 20,000 Vulcans at the time of the invasion.
Conclusion: The Romulans will roll over the Vulcans and conquer New Vulcan.