In my opinion you will do much better and be happier with a decent vise rather than a rigged contraption made with clamps. While the latter will certainly work in a pinch, the cumbersome mechanics of using it will wear thin very quickly.
For my first bench I bought a vise off Amazon for $60 and made my own jaws. That vise will probably outlast me. I think too much is sometimes made of things – like the Veritas vise. They certainly aren’t the end-all/be-all. They’re nice – but lots of things work very well – some would even say better – and cost a whole lot less.
Another one to consider is the 9″ Eclipse vise from Highand – maybe Lee Valley…can’t remember which.
That’s my two cents worth anyway!