The first thing I recommend with any water damage situation is to open the product up and make sure you don't have corrosion building up inside. You may need to clean it with isopropyl alcohol.
Follow these [[Electronics Water Damage|water troubleshooting directions]].
The power to your vibrator is obviously still working, so it may be a quiet switch that is broken. I don't have experience with this particular phone, but perhaps check the software settings and see if you can manually trigger the vibrator.
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