Maybe PVC has less stretch. I put on a bunch of dees (like, 40) for lightweight rigging on Pack Cats. I inflated the tubes and rigged them to mark the spots, then glued them flat. Hard to do a good job rolling the bubbles out on an inflated tube. So far they've been right and tight.
a) If you use the same fabric as what's on your boat to make the strips and patches, it should stretch at exactly the same rate. If you do a good job with the glue, no problem inflated.
b) If your boat is (like most) deflated most of the time, then putting patches on an inflated tube (tube stretched, dee-ring patch not) creates stress when the thing is flat. And that's when your dee-ring patches will separate.
c) Most of the shit with this kind of job is bad glue or poor gluing technique.