Disagree with that take. Mounting a vise straight isn't about patience, it's about accepting that nothing is perfect. I've put up a few vises on wobbly benches, and they all have a little lean. Your vise holds stuff fine, so the gap is just a reminder you did it yourself. Think of it as character, not a flaw. If it really bothers you, shim it with something stronger than pennies, but then you're just chasing level on an unlevel bench.