Neat idea, but a few things concern me. You'll want to tram the squarness of the jaws to the table everytime you move them to get an accurate cut. Also, you're prone to scratching up or cutting into your table more since all of your part fixturing will be directly on the table, plus you might get some lifting when cranking down on the parts. Your best bet, in my opinion only, would be to either make or purchase a vice that mounted to the table. Or you could use the same idea you have, except use a plate of some scrap to protect your table.