Perhaps this invasion was just the first wave, with the Romulans trying to establish a beach head somewhere on Vulcan. How effective this tactic would be would depend on a bunch of logistical concerns: how strong is Vulcan's planetary military, how fast can the Fedeation mobilize an army on Vulcan, what equipment the Romulans brought with them.
One logistical question, do the Romulans have assault vehicles that would allow them to outflank the enemy? You almost never see any sort of ground vehicles, how troops supposed to outflank the enemy without ground vehicles?
I do think 200,000 soldiers would make for a better beachhead, then 2000 would, though. I'm not sure 2000 soldiers would be enough.