The milling vice I use was made by a friend who ground tools for a living, its very well made dead square and parallel and hardened, as it was intended for use on a magnetic grinding table it has no holes for bolting it to the milling table. Some time ago I fixed a couple of pieces of 1" square mild steel to the side for clamping.
Like this

Over the last few months I started to realise that it was not too convenient for setting the vice square when putting it on and off the mill, so I decided to move the clamping holes so that I could fix some location buttons on the base of the vice.
These are the buttons turned to be a nice snug fit it the T slots and tapped M6, I put some centre pop marks on their base so that they would bite and allow them to be tightened up.

This is where they go

Just nip the buttons up assemble the vice to the table, square it up with a DTI, tighten the buttons up good and tight.
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Now when I put the vice on it will be dead square.
Thats another one off the list.
Stew