First of all apologies if this has already been posted, but for years i have spent many fruitless hours gettig it to be true to the x axis.
I saw this somewhere, and it is soooooo easy once made. First of all get a piece of MS bar that will fit tightly in the bed slot and stand proud about 0.25in above the bed.
Then get the vice and turn it upside down and clamp the jaws onto the bar. Make surethe the bed of the vice is level in both planes.
Then milla slot equal to the width of the bar in the slot and as deep as the bar protrudes above the bed, again make sure by testing with a spare length of bar that it is a tight fit.
When the vice is turned correct way up you will find that the jaws a parallel to the x axis and can be removed and replaced with spending hours setting it up.
Hope this makes sense!