You will need an upper receiver action block or a barrel extension torque tool. (Let me know if by some chance you live in the Phoenix/Valley area of AZ as I have both)
You will not crush your barrel with the barrel jaw blocks, but yes you can break a vise by over tightening it.
Another possibility is a free float tube that does not require you to remove the barrel nut. Midwest Industries 2 piece and the Samson Evolution come to mind. There are others.
http://www.brownells.com/.aspx/pid=2...R-ACTION-BLOCK
ETA: You can also wrap you barrel in a thin lead sheet and/or use rosin to try and keep it from slipping. I gave up on barrel blocks except for very limited uses. At that point I normally use a real barrel vise mounted to the floor.
http://www.brownells.com/.aspx/pid=2...ON-TORQUE-TOOL