I always wondered what they long term plan was. Let's say they did conquer Vulcan. Then what? They are in the middle of Federation space and completely surrounded by Starfleet. By invading Vulcan they essentially declare war (they'd have to cross the Neutral Zone to get there) with the Federation.
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.
But maybe they were looking to start a war for whatever reason. Who knows? And maybe, like it was theorized earlier in this thread it was just Sela's superiors way of setting her up to fail.