As a projectile moves down its vertical velocity is positive?

When YOU start working on a problem, if the question doesn't TELL you which direction is positive and which is negative, then YOU get to decide before you start solving the problem.

MOST often, people use positive for UP and negative for DOWN, just because that's the way we naturally think of 'up' and 'down'. But there's no law. If you want to, you can just as well call up 'negative' and call down 'positive'.