Is it safe to use a vibrating chair while pregnant?

Yes: Yes, you will probably get more vibration from walking around and sitting in a car than from sitting on a vibration chair. The baby is protected in fluid to help with outside vibrations.

Vibrating chair: Not sure what kind of vibration such chair will do and what the reason behind. To avoid having unrest in your mind that such a thing may harm you and your baby, i'd avoid it unless absolutely necessary.