I don't think the situation is really worth preparing for honestly, but to dispell a myth :
You can discount any handgun round defeating a decent ballistic vest.
This is 100% not true. I have personally taken vest that have seen just year of road use in my agency and shot them.
A hot 9mm cut through a 2A like butter.
Most don't take care of their vest properly and weaken them by folding, rolling etc...
Instead of concentrating on defeating a vest I would concentrate on dumping energy into it. It's not like in the movies people don't just get back up. If you dump 500 or so ftlbs into a vest, if it penetrates or not, it's going to break ribs, knock the wind out of them, cause temporary loss of body function, etc....
In your above situation the BG might have got of a shot regardless.