The usual rationalizations:
1) The plan was to conquer and hold Vulcan for X number of days, for propaganda reasons only. 2,000 troops were plenty enough for it, as history shows us that troop requirements go way
down as technology evolves. The plan failed. The villains told the heroes that this was what they intended, and then let them escape - which makes it the least
likely alternative to actually be true.
2) The plan was to conquer Vulcan for good. There were no troops aboard the old Vulcan ships - all the hundreds of thousands of troops were in the dozens of cloaked warbirds that easily penetrated Vulcan space thanks to all the noise that the old Vulcan rust buckets made. This plan failed. This is also rather unlikely to be true, because we get no good reason for why the Romulans would admit defeat here; in this model, they clearly had the upper hand with their cloaked fleet, and could (and should) easily have destroyed the E-D and the other ships defending Vulcan.
3) The plan was to pretend that Romulus was about to conquer Vulcan. To this end, Romulan peaceniks were invited to board those old Vulcan ships, and these peace envoys were then shipped to Vulcan while Romulus allowed Ambassador Spock to spread the false rumor that an invasion was ongoing. The peaceniks were then executed in a maneuver that made every UFP adversary gasp in awe and respect of Romulan audacity (plus snicker at the stupidity of Starfleet). Since this is the exact opposite of what the Romulans told our heroes before letting them escape, it's also the one the most likely to be true.
For all the alternatives, we can also freely speculate on how many Vulcans would gladly welcome their new Romulan overlords and be happy to get rid of Federation scum.