A few weeks ago I showed you a handy dandy jig for keeping the jaws of your end vise parallel. When you clamp a piece using just the corner of the vise, the jaws want to twist and won't remain parallel. Using this jig alleviates the problem no matter what the width of the piece you are clamping.
The jig is just a number of 2 x 3 inch pieces held together with a bolt and a wing nut.
Swing down the number of pieces to match the thickness of the board being clamped.