Glad your back's a little better, Mike, and let's hope this continues and your agony diminishes swiftly.
I've not had to resort to the vice yet, but like you say, some nice friendly jaws and, I guess, clamp on those flats near the breech opening. It won't have to be TOO tight, as long as it's held and can't rotate.
Might just be worth another go before trying the vice, with yourself (or your helper) gripping the action with some padded gloves. Tight fitting sturdy bar (whilst protecting the end block) with at least six inches protruding so that you can shock the bugger off with some solid whacks with the hammer.