You shouldn’t need to crank down on a bench vise to hold something securely. To improve the vise’s grip, you can line the jaws with leather. The cushioned surface gives a little and helps create a tight hold without a lot of pressure. As a side benefit, leather is less likely to mar a workpiece.
Another thing to do is make sure the clamping pressure is balanced across the width of the vise. You can see what I mean here. I made a spacer block to keep the jaws from distorting. The block has steps cut in it that match up with common workpiece thicknesses.
If your vise doesn't close evenly, it may be the result of improper alignment of the vise or by repeatedly clamping workpieces on the same side of the vise. Over time, this can distort the position of the jaw so it’s no longer square to the front of the bench.
First, check to see if the mounting screws are loose. If they are, realign the vise and tighten the screws securely. If your vise has a metal rear jaw, you can loosen the mounting screws and slip shims behind it. Your goal is to make sure the vise holds the workpiece evenly across the front of the bench. Retighten the screws and you’re done.
A second, and possibly simpler, option is to adjust the shape of the front jaw so it's parallel again. Use a compass to scribe the front jaw so you know how much material to remove. Use this layout line as a guide to remove the waste from the jaw. You can do this with a planer, belt sander, or even a hand plane.
