Beat vise racking with wedges
Wide bench vises tend to stray from parallel when you snug up an object in just one end of the vise. To get the upper hand on that situation, insert a pair of opposing wedges into the other end of the vise. (You may want to add a cleat to the bottom of the vise jaw for one wedge to rest on.) Then, as the vise opens, the opposing wedge slides down to its correct position. Make wedges in pairs of varying sizes to accommodate large and small clamped objects.
—Matt Olan, Spruce Grove, Alta.