Back in front of the computer, can reply better.
I would urge you not to be tempted to use something like this:
It is not that hard to rip the pins through the upper. I have one to use for stuff like clamping my long 26" upper in a vice while I clean the bore, but they are sketchy for twisting stuff. You would probably get away with it for a muzzle device, unless you don't.
One thing I have done is use a couple short pieces of automotive heater hose, slit lengthwise to cover the vice jaws (or wrap around the barrel), and that might work.