Regarding silk screened backs:
It might be that a screen printer in your town can remove that image. I used to work at a screen print shop, and that's how we fixed our mistakes. There's a fluid that shoots out of a high-pressure spray gun and it dissolves screen printed ink.
Another option might be to take one of those waistcoats to a good seamstress/tailor and have them take the back apart and turn the printed image to the inside.

Goodwill's probably your best bet.