Four square milling

This may not be the right forum but I can't find another one which covers my question.

I have an FP1 and am having difficulty in machining a truly square 3/8" bar of aluminium.

I think it is the vice that is the trouble, it was second hand and is very good quality, but old.

There are precision vices which are angled so that lifting is eliminated. Is it worth spending money on a new vice?

Keith