Be sure to go to a street party!!
The one thing I loved about St Vincent was the street parties. Even though I was there well before carnival there were still lots of parties - be sure to check out Calliqua (southern part of St Vincent) which is a small village known for street parties - Friday nights they have one called Culture Jam - there is a dj and lots of rum and dancing. They often hold soca nights on sunday nights as well in May/June across St Vincent - each sunday in a different town. It pays to get to know the locals though, as much of this is word of mouth!