I'm curious why you need to paint the vinyl? It's made to be printed on so why not just do that? The paint is much thicker and would block the holes of the perf I would think but I've never done it so who knows...
What colour do you need it to be? 3M sells a day/night perforated vinyl in black, blue and printable. Also if it's a channel letter why would you need a white vinyl layer? You would just use white acrylic for the faces and save yourself the extra step.