If it was free, and a regular washing didn't solve the problem, I'd say you are at the point where you may need to take a chance on ruining it by using a more aggressive cleaning method.

I can't imagine a vinegar solution would ruin it, but the cure may be nearly as  bad as the problem.  It might turn out OK if you let the pack air out fully on a few hot sunny days.  Baking in the sun seems to help dissipate a lot of odors.  Once you're done and you want to know how it will smell to Mr. Bear, just put it in a sealed container and then take a deep whiff when you open it up a few days later.

Hopefully you'll end up with something more useable than a free jumbo-sized barf bag.