Once this is determined to be truly a hardware problem rather than software...claim warranty. If warranty does not exist, here's an option:

This happened to me once -- I had to take apart the BlackBerry. The vibrator area was full of pocket lint. I just blew that off and reassembled.

Please make sure your BlackBerry is out of warranty and you're comfortable doing, say, a computer upgrade (opening up a computer and then putting it back together). The difficulty is only slightly more difficult than that, since you don't need to do much disassembly before you can blow lint clog away from the vibrator.

It can also be a broken wire or dirty contacts. Clean the contacts with a Q-tip. (Wire for vibrators are rare now due to metal fatigue, but if you see one, it can be simply resoldered back using a fine-tip soldering iron)