I usually scrounge a couple of stout parallels, perhaps a couple of pieces of 3/4" x 4" flat bar set on edge. They should be at least as long as your vise is wide. Set them inside the vise, extending out very near to the blade. Put your piece of stock in position between the parallel extensions, and use another piece of your previously sawn stock for a spacer at the opposite end. Tighten the vise and check that both pieces are snug. Saw away.