I have two different machinist vises that I use on an x2 so here is the way I see it. I have a small two inch one from
www.shars.com and a 3 inch wide one from Enco that was gifted to me. Both are very different(even more than size). The three inch wide one from enco works fine on the x2 as far as size wise. The main thing I do not like about it is the fact that you cannot ratchet it. to move the jaws a notch, you have to take out the pin and then move the jaw and hit the hole blind. This can be frustrating because you use the same holes to hold down the vise and invariably the hole you want is where the hold-down is. Other than that I like the vise fine.
The vise that I have from Shars is different in that it uses the a ratchet underneath and you can loosen the bolt and change the positioning fairly simply. Also, the hold downs do not get in the way. The only problem with this vise is the nut that holds the ratchet underneath. The nut was made a little too small and when tightening the vise, the dowel can twist and slip. This can be fixed by machining a larger nut, but I have been too lazy to do that.
Both vises are accurate enough and I have no trouble mounting them on the x2. One thing that I would note is that if you buy a 3" or 3.5" vise, make sure you get the 3" parallels from LMS. The 6" parallels will forever fight you with the hangover they have trying to put your part in the vise. They seem to always be trying to flip my part out of the vise.

Just my $.02
Anthony