I'm wondering if reprogramming some used FPGA miner to generate vanity addresses will be possible.
As long as they didn't lock out programming on the FPGA, sure. That applies to any FPGA product, though - and at this point unless you're getting a sweet deal on an old FPGA miner, you'd be better off getting a dev kit from one of the big FPGA companies or their partners.
Before couldn't use my gpus to mine addresses with due to my cards using this software I am able to generate with GPU but slow. Still least I can now generate than before I couldn't.
I strongly suspect that's down to the command line used, then - the
vanitygen binaries in the bither package (Windows anyway) are stock
Vanitygen 0.22. Might want to try figuring out what command line it uses, then feed that manually to vanitygen and try changing parameters around from there.