Would I be able to paint over vinyl with acrylic paint, and then go over it with polyurethene to make a watertight seal?

It's a decal, but it's not colored. All I need is white and a few touches of gray here and there (it's of Baymax), but I don't want the paint to chip off or anything. What would be the best way for me to paint in the white and still create a watertight finish that blends into the vinyl? I wish I didn't have to do this, because I mean, Baymax himself is made of vinyl, but I really want him to be colored.