I think that I mentioned buying a very nice toolmaker's mill vise from Little Machine Shop. It is a bit bigger than the 2" Sherline mill vise, and clamps
very solidly, but the question was, "how to mount it to the table?"
The mounting slots were just a bit too high and too far forward for the Sherline toe clamps. So, I made a set, starting with angle bracket.
I trimmed one leg to .52" height, which clamps parallel to the mill vise's mounting slot and still is square to the table, then I drilled .203" holes to run 10-32 screws to the T-nuts in the T-slots. I was at first concerned that only having two attachment points would fail to adequately secure it, but it appears that two are enough, at least for the depth of cuts that I can take with this level of power. I might make longer top clamps that can screw into T-nuts in both T-slots. This is slightly harder to do because the most back part of the vise's mounting slot is ahead of the back T-slot. I will probably use a 1" angle piece, and remove 1/2" on the top leg, to allow a 10-32 screw hole for the back T-slot.
A learning experience: you really do want to use a 4-flute end mill on steel. I have mostly used 2-flute end mills until now, which work well in aluminum and acetal; not so fast in steel. This 4-flute mill was also a roughing mill which I am sure helped.
And I will make those extended toe clamps. I can feel a few thousandths of an inch of misalignment where the back of the vise matches the mill table. This is after milling a number of .001" depth of cut on acetal at 10 inches/min. This is not a very heavy demand. Using four clamps will enable me to spread locking force across twice as much area.