Still working in vB 3.8.1 flawlessly. I had to update my templates so I basically had to re-install this hack because I accidentally overwrote the template edits for this hack to work.
Follow the above instructions in the first post. You do NOT need to edit the MEMBERINFO template.
Edit both postbit and postbit_legacy templates with the instructions above for postbit_legacy. Also follow the instructions for the USERCP templates...
Then set the default signature in vB settings > Default signature and you'll see your default sig go live.
Edit: I nominate this hack for MOTM.