Phone not vibrating after replacing back full housing?

I would try the following, and test after each step:

Step 1: reboot the iPhone by pressing and holding power + home button together for ~12 seconds, then restart and test.

Step 2: clean and reseat flat cable:

Flip up the screen, look at where the power and audio control flat cable attaches to the logic board (next to the screen connectors).

2-a/ Disconnect the 2 interconnect cables and the audio control cable as per step 21 of this guide. Clean the connector on the board using a cotton swab dipped in 90%+ isopropyl alcohol.

2-b/ Reseat and test.

Step 3: pry damage: if no change after doing the above steps, look around and left of the connector (same area as in 2-a, as per step 21 of the guide). Is there anything missing/damaged/broken/part removed? There are a number of components there that affect the function of the vibrator, and they can easily be pried off when disconnecting the cables.

Step 3: flat cable damage and/or improperly seated vibrator: remove the battery and logic board, examine the power/audio control cable. Is it torn? Severely creased? Is the vibrator properly seated and touching the two gold prongs on the top left corner of the phone? If you can and have to, try a different cable. Let us know what you find out.