Quite a few possibilities there.
1) The old Vulcan tubs were shot down by a cloaked warbird. The invasion army would no doubt ride on further warbirds, and might well number in the hundreds of thousands; the Vulcan ships would only be included in order to produce so much noise that Federation sensors wouldn't notice the cloaked armada.
2) There never was any invasion army. Romulans just packed their peaceniks in Vulcan ships, just as they told the Federation - and then killed them all with the Federation's full blessing, when their usefulness as a decoy had expired. The whole galaxy would respect Romulan derring-do even though their "plan" didn't quite succeed; a major double propaganda victory for Romulus, then.
3) There never was any invasion plan - just a plan to destroy Vulcan once and for all. The "troops" didn't exist, except as sensor ghosts, and all that was going to be delivered to Vulcan was a doomsday bomb of some sort. Or then that armada of cloaked warbirds.
4) Invent your own devious plan here. The very fact that Sela told Picard, Data and Spock of this "invasion plan" should be proof enough that no such plan really existed...