First, vyvanse is a med that can be mixed with water.
It does need to be taken fairly soon afterward, and the whole capsule mixed into the water. Your child would need to drink all of the water it is mixed with. If he only drank half the glass, he might only get half the dose. As far as I can tell, the amount of water used is not a problem as long as all of it is drunk.
Apparantly, acidic foods can cause vyvanse to not work as well.