Hi,
after you check if there is no setting that is preventing the vibration you’ll need to reopen the phone.
The vibrator has to simple coper connectors that are pushed to their counterparts on the motherboard. clean these connectors and check the flex cable. Make sure (at reassembling) that these make a good connection. Peek under the motherboard a little.
After reassembling, test if the function works.
Btw, replacing the battery should not have an effect on the function of the vibrator.
Kind Regards, Brenda