Don't do it! If your already experiencing moister it will probably make matters worse. It's like drilling a hole in a leaky boat to let water out. This is because it lets the warmer humid outside air into a cooler space inviting condensation. It may even be that you moister problem is because of your existing vents.
First eliminate other possible moisture sources. 1/ Is water leaking through your foundation. 2/ Is there a ground source vapor barrier. If these are existing, than the next source is condensation. Your solution would then be sealing what vents you have.