1. The primary purpose of a cock ring is to prolong and strengthen an erection. How well it works mostly depends on how it fits. Presumably by making this happen there is some pleasure involved, but it is mainly about the keeping that erection, in my opinion.

2. Some vibrating cock rings serve the purpose of a cock ring and some are just a way to attach a vibrator to a penis. Again, it depends on the fit of the ring. If it is too stretchy it isn't going to work well to prolong an erection.

3. I don't think I have ever worn a vibrating cock ring just to pleasure myself, but there is no reason why a guy couldn't. However, making a masturbation session take longer is not usually the point for me, I usually am just going for a release.

4. The testicle ring on cock rings that have them can certainly have an impact on ones erection. It might not be as pronounced as a ring that goes around the shaft or around both, but it still makes a difference.

5. A cock ring that does not stretch can still work as long as it fits well. It can take more effort to get on, and may be more difficult to take off, but the fact that it does not stretch can make it very effective if it is sized well.