I like mechanisms that are smooth and effortless to use. And I definitely want my leg vise to work that way. After building brackets to hold my skateboard rollers, it looks like I've achieved that. The results are just what I was hoping for. The vise chop moves in and out in a perfectly straight line all along the length of the parallel guide. The chop is completely supported by the parallel guide and the rollers, not the vise screw. So the screw will turn smoothly since there's nothing pulling it down.
Here is one of the two brackets I made to hold the skateboard rollers. I used the smooth portion of a 5/16" bolt for the roller shaft (I cut the head and threads off). I locked this in position with a couple 10-24 set screws threaded into holes tapped in the flanges of the bracket. The roller is kept from rubbing the inside of the flanges by a couple washers placed between the bearings and the flanges.
The brackets attach to the legs with a couple 3/8" cap screws, tapped directly into the legs.
This is the rear bracket, on top of the parallel guide.
...with the parallel guide in place. I positioned the cap screws so there is about 1/4" vertical adjustment room.
The front bracket.
The same bracket with the parallel guide and chop.
View from underneath.
Here's a video showing how smoothly and easily the vise chop moves.
Next I'll try to devise a similar mechanism for the sliding leg vise. This poses a problem since the sliding leg vise has much less space below the parallel guide.