I had noticed during my bandsaw project that my milling vise had a little bit of play in it.  That is, the movable jaw was not clamped securely to the base.   There was about 0.1-0.2mm (5-10 thou) of play, and the net result was that when you clamped a piece in the jaws, the moveable jaw would lift by that amount as it snugged up against whatever was being held.  this meant that the piece being clamped would lift slightly on the moving jaw side.  Much frustration ensued, and I was saved only by the fact that the bandsaw project didn't actually require that much precision.  Well, with that project behind me, I decided to tear down the vise and fix it!

Here's the movable jaw clamped to my mill's table.  Notice that the jaw is clamped right-side-up, but what I really need to do is remove some material from its bottom, as that is where the clamps that hold it to the base fasten.  It'll become clearer later on…  At any rate, my first problem was that I didn't have a pair of surfaces in the same plane (parallel to the jaw's base), so that's why it's clamped here as it is. 

Next, I shaved down, every so slightly, the flat on the rear of the jaw.  I'm doing this with a fly cutter, and I'm only taking off about 0.025mm (1 thou).

Then, without moving the quill or the mill's head, I whip over to the other end of the jaw (note that I had to reclamp from the other side of the jaw, adding the new clamp before removing the old one) and  knocked it down to the same height.

And here's the vise jaw with the top-side machining complete.  Now I can clamp it upside-down on these newly machined faces and be certain that the underside is parallel to the mill's table.

So, I clamp it to the table upside-down, and shave about 0.25mm (10 thou) off of the two clamp supports.

Finally, I put the vise all back together.  Now the (black) clamps hold the moving jaw securely to the base.  The action is still smooth, but a little stiff.  Happily, play is completely gone.

I think that I will shim with some thin pieces of paper between the jaw and the clamps that hold it to the vise's base.  That will allow me to tighten the screws not quite all the way, while still making them tight enough that they won't come loose.  At any rate, this was a pretty easy job to do, so if any of you have similar play in your vises, have at it.

-Chris