Shutterbug-
I've done exactly the same thing as you with the same materials on glass. Unlike you, though, I've applied quite a lot of other vinyl, so the air bubble part under the vinyl was not as much of an issue. Sign installers use slightly soapy water or a ready made product for the same purpose such as Rapid Tac. I think there is a surfactant in it and something that makes it dry slightly faster than plain water.
You are correct, that the proper way to protect the vinyl is with a clear overlay layer, but I don't have a laminator either. For the pieces that I printed myself on my Z3100, I also used many layers of Matte Finish. I wasted some prints doing that, though, as I think I applied to many coats too fast. I've used a lot of Krylon before, and the layers weren't applied too fast for the clear finish itself, but the combination of vinyl, ink, and Matte Finish was finicky. When I had issues, it formed air bubbles that I assume were from the outgassing of the ink.
Regarding longevity, I only had my prints on an exterior window for about three months. When I went to pull them off, the ink and Krylon had dried out so much that it completely flaked off of the white vinyl when I pulled on it. I was left with ink flakes on the ground and a crumpled wad of completely white vinyl scrap.
Best of luck,
Ron H.