I will use the bench vise on occasion to hold the bbl for breech plug removal, but if it doesn't come right out, I move over to my bbl vise.
In the bench vise a set of wooden jaws w/a V cut lengthwise will suffice most times to hold a M/L bbl secure. You need something other than just the vise jaws clamping down on the opposing flats of an oct bbl in most cases. The bbl will roll right out of the bench vise on you with likely damage to the bbl otherwise.
Wooden blocks/jaws made up specially for the bbl as described above work the best. Cast the bbl profile in epoxy. Use a bit of rosin on the round bbls (sugar works pretty well too) to prevent slipping. Holding the bbl upright or at an angle in a bench vise with these block jaws works good as the vise jaw spans the block.
Sometimes I cast the blocks from lead. A simple box mould made of turned up copper sheet w/a cut out at each end to allow the bbl to lay down in the 'block' a bit. A simple pour of two of these for one bbl yields a very nice set of never scratch jaws.
Set them down less then 1/2 their dia so you have some draw avail when in use.
Pure lead is too soft I've found as it extrudes when tightened down in a bbl vise,loosening as you work.
Regular bullet mix works fine.
A bbl vise is easy to make w/a couple pieces of 1/2 to 3/4 flat stock steel and 4 sturdy bolts to draw the top plate down upon the bottom plate w/your wooden block jaws in betw. Bolt the entire thing to the far end of your bench facing lengthwise. Out of the way but with the room to use it when needed.
A wrench of two pieces of 5/8 or even 1" square stock and two bolts. The 'handle' about 12+inches long,,the jaw piece about 5". Make the holes in the jaw piece loose enough so the jaw can be tightened down at an angle onto the slant sided breech plugs and other less then square shapes.
Keep a cheater bar of a piece of steelpipe to slide over the end of the wrench handle if needed to break things free.
A piece of copper or brass between the plug and wrench prevents marring.
With the above wrench, bbl vise, wooden & lead jaw/blocks and a few odd adaptor plates and shims made as needed,, over the years I've removed and replaced countless center fire rifle bbls Military & commercial, shotgun magazine tubes, bbl extensions, pistol and revolver bbls,,and M/L breech plugs.
Simple hook wrenches, cresent tools and the like can be used but they usually mar the breech plug up and just don't have the control of a tight fitting tool like the above. You have to put the plug back in later on and would like to have the same sturdy set up to ease the plug back in to it's witness mark.
Just some thoughts