Portable Bench Vise

Assuming what you mean is 6” thickness,the only one that comes to mind is (https://www.japanwoodworker.com/products/carvers-patternmakers-vise) or the like. Unfortunately the website illustration isn’t working currently. For a similar one with shorter bolt see (https://www.woodcraft.com/products/woodriver-patternmakers-or-gunstock-carving-vise). (There are a number of these on the market from various tool suppliers, not all the same length of fastening screw.) You’ll have to bore a 5/8” or larger hole to accommodate the bolt that clamps it to your work surface.

You can get one from Harbor Freight.

(https://www.harborfreight.com/6-in-swivel-vise-with-anvil-63189.html)

 

 

I take it that by a portable vice you mean something that clamps to the edge of a tabletop or bench.  If that is the case and there is anything sharp involved, whether it is in the vice or your hand, my advice would be don’t.  Back in the days when money and space was limited, I went through several renditions of these portable vices and have the scars to prove it.  They are not stable, they will twist, they slip, they move.  Some of the worst money I have ever spent.  I lost a lot of blood due to these. 

I usually bolt the vise to a board, then I can use whatever clamps work to hold it wherever you want. This gives you the opportunity to buy a Red-E-Hand vise, the best of all possible vises.

 

If you can find one of the older Zyliss 4in1 Vises they are great.  Evidently in recent years they have been sold and newer versions are made in China instead of Switzerland.