Drill a hole through a piece of wood, that is as thick as where you want to grip or more, a bit smaller than the grip area. Then saw the piece longways through the center of the hole. Now you can grip the chuck in a vise without damaging the chuck!
Don't let it put you off them but bear in mind that most of them will tighten when loaded. A large dia holesaw has quite a bit of leverage so will make the jaws tighten up. Also if your saw arbor was hexagonal as most are it will just keep putting more load onto the jaws whereas a drill bit will spin in the jaws.