St. Vincent is an EC (Eastern Caribbean dollar) country whereas Barbados has it's own currency, so unless the ATMs have the option of dispenseing in US I would wait until getting to St. Vincent, I havn't been to the St. Vincent airport but someone should know if they have ATMs there. US is accepted evrywhere, but the street exchange rate isn't as good as the bank rate.

Drink all day at home, your friends worry about you; do it on vacation and they say "what a good time you're having". Save your friends needless worry, travel more!