Light cuts or you may get some chatter. Also, when cutting, come at it from the other direction......I always like to have the cutting force directed toward the fixed jaw.
Better would a large angle plate bolted to the table and clamp the work to the plate. Light cuts until you get two square sides, the tuck a parallel under the work.
The other thing you can do, if you have one that is square, is place a large block between the work and fixed jaw. A 4x4 hardened ground block or similar would work well, or a 2x4x6 block. Then clamp the work to block. This gives the work a little more support.