I teach and take metalsmithing workshops. Often these are not offered in fully equipped studios, and I need to supply substantial vise that will clamp down to a table.  I have found the commercial jewelry clamp on vises woefully inadequate. Watch the sales at Harbor Freight or an eye out at yard sales. $30-$40 new or $5 to $15  yard sale find, a scrap of plywood, a couple of C clamps and bolts will make you a strong useful tool you can use for years. You might find one to "steal" in your backyard toolshed/ workshop. I admit to borrowing ours  and then just drywall screwing it to workbench when it comes home. Check out these photos for details and send me a pic of yours if you make one!