Perhaps this will help.
Well, here is the deal.
I just recently had my iPhone replaced for the third time during my warranty period. This third phone is just as messed up as the others, except now it presents a new problem. The vibrate seems to die... it just isn't strong. Until that is... I go into the settings and discover the sound level has turned itself down. Odd right? It goes to about 3/4 the full level. As soon as I turn that level back up, then put the phone back onto vibrate, everything is perfect again. It is really stupid.
Here you go. Try it. Good luck.
Jeni