I've seen it suggested that an effective and simple device for vice alignment can be made from two shallow right angle edged trays built up from modestly thick plate. I imagine selections from something between 1/4 and 3/4 alloy would work.
Assuming a rectilinear table the first U sits on top of the table with its arms projecting downward in line with the bolt down slots. Studs and suitable hand nuts aligned with the slots being fitted for securing. The vice is fixed to a second U sat on top of the first plate with the downwards arms running the other way along the plain sides of the lower plate. Slots and studs being provided to lock the two plates together when the work is aligned.
Clearly one plate lets the vice move in and out and the other from side to side. Fundamentally a crude approximation to a cross vice. Arguably more rigid when locked down and little harder to operate if things move smoothly.
Alternatively both U sections could be on the bottom plate. One set up and one set down with the vice on a simple flat plate. If the U shapes were made with narrow keeper strips on top to prevent lift this arrangement might well make hand holding safe and satisfactory for all but the heaviest of jobs. Having no locking studs should make for faster operation at the cost of ultimate security.
If you have a round table with radial Tee slots it would almost certainly be necessary to add a rectilinear top plate to make this scheme work. I can't see the Tee slots providing sufficient range of guided movement.
Whatever you do it has to be quick and easy for you to use. As ever the best approach is to find out how many different ways have been sucessful for different folk doing different jobs with different equipment and select the one that works best for and your work on your machine. Then fine tune it and tell everyone that you've got the perfect answer.
I imagine that, after 35 years, Sam has gotten really quick at the table movements needed to get his vice in just the right place. Moi, being a lazy toad, would have been looking for an easier way almost before getting the drill wired up in preference to settling down to learn how to make the table shifting way work!
Clive