Maybe Romulans believe that the average Vulcan would support them? It sounds crazy but there are historical parallels. During the Bay of Pigs invasion of Cuba the US Government and Intelligence services were convinced that news of an attack by a few hundred ex-Cuban nationals would trigger a mass anti Castro uprising.
I've also heard (though I can't confirm the accuracy of this) that in the late 70's a group of Argentine civilians set off to liberate the Falkland Isles. Having heard the official account of how the Malvinas were Argentine sovereign territory, now labouring under British Imperial Aggression, they were very surprised on reaching the islands to find the locals speaking English, and not wanting to be 'liberated'.
So perhaps the Romulans hoped that their troops would be reinforced by members of a vast pro-Romulan movement.