The Vapor cases make GPS unusable b/c it blocks the signal. Also if you live in a weak signal area you're screwed b/c the Vapor degrades signal by about 20db. I had one for 3 days and sold it. As for the knock offs... I would think any metal band around the phone is going to give you issues.
Yeah knock offs are not usually good quality to begin with. The vapor case like Poseidon79 said gave a lot people problems with signal. I would try something different that maybe doesnt have a metal band.