According to a website I found (http://www.recipezaar.com/library/getentry.zsp?id=175
) there's no calories or sugar in a vanilla bean, so I'm guessing it wasn't additional sugar in the bean that restarted it. My guess would be maybe the fermentation was stuck, and adding the beans stirred things up enough to restart it. Another possibility is some of the CO2 stored in the liquid was released when you poured the beans in. Not to scare you, but my third guess would be an infection. How did you clean the beans before adding? Anyway, the best clue to solve this mystery would probably be a gravity reading.