There is another solution that doesn't mean butchering your vice. Try a vice extension. They are an L shape that fits into your clamp and lifts the vice (or lowers it if used the right way up).

Without seeing your tying position it is difficult to say what the cause of the pain is, but usually it is from holding up your arms. Higher usually means holding your arms higher. Lowering it, and resting your arms on the chair arms might help. It does help me.

Cheers,
A.