What is wrong with using the keys in the bottom of the vise? Granted, you may have to accurize them, but it is well worth the trouble, IMO.
Set up a 10 or 12 inch long straight bar, clamped directly to the machine table, and dial it in to perfection. Take your vise, invert it and clamp the jaws directly onto the trammed bar.
Now check the existing keyway slots for accuracy and touch them up if necessary, or cut new ones in a different place if that would be handier for your particular machine.
Make some close fitting keys that are tight in the vise keyways, and about .001" loose in the mill table slots. You might need to make step keys to accomplish this and you might also want to install them into the vise keyways and take some light truing cuts on the oversize keys with the vise upside down as described earlier.
I've done this for my Kurt vises and it works well. I simply place the vise in a convenient T-slot and clamp it down lightly. The .001" clearance means it could be out that much in 6 inches, so then I adjust it with a soft hammer, one way or the other. I'd be ready to go in a minute or so.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)