I had the same issue. I had to open up the phone to fix it, but didn't need to do much more than actually opening it.
Bear in mind this will void your warranty.
Here's what I did:
Turn off the phone.
Remove the SIM and SD card trays.
Take a guitar pick or thin plastic object and gently (but with a small amount of pressure), pry up the black plastic piece on the top of the phone, it's held down with some adhesive, so you might need to peel it a bit.
You will find two size 4 torx screws under there, one on each side, recessed into the housing. Remove them.
Now use your plastic implement to start peeling off the back of the case. It works best if you dig it into the seam, then once you have it in there, run it around the edge to release the clips holding the case together.
Once the back is off, you will find the vibration module in the upper right of the circuitry you find inside, it's a small black cylinder with a silver head on it.
It's held in place with rubber bumpers on either side. Take your plastic implement and gently lift it from its housing.
Now rotate the head a few times between your fingers to loosen it up, also giving the head a very gentle push and pull - VERY GENTLE.
Now put the vibration unit carefully back into its housing.
Clip the rear casing of the phone back on. This can very tricky to get all the clips to snap back into place. Don't do what I did and use pliers. My phone now has a small dent in it. Whoops!
Once the rear casing is back in place, replace the two screws in the top of the phone, followed by the black plastic piece you removed at the beginning.
Power the phone back on, and with a bit of luck, your vibration unit will be unstuck and working again.
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