How could 2000 Romulan troops take over Vulcan?
With the assistance of many millions of Vulcans who sympathize with the Romulans way. Who wish for a re-joining of the two branches of Vulcanoids. Who see this as the beginning of a path to peace between the Federation and the Empire. Who long to cast aside the restrictive philosophy of Surak. Many different groups would welcome the Romulan "troops."
The troops were less infantry, and more agent provocateurs. There not to fight a war, but to ignite the revolution that various segments of the Vulcan population have been waiting for.
That was the Romulans plan.
Romulans don't take a dump ... without a plan.