I got a matched pair of Vertex swivel base vices for my Lux style mill and haven't regretted the expense. Especially since moving up to a Bridgeport which has room to use the pair on long work. That said, although the swivel facility is very useful at times it doesn't get that much use overall, maybe 1 in 10 to 1 in 20 jobs, so on a smaller machine its arguable whether the convenience of simply swivelling the vice over more complex on the table set-ups ware worth the loss of vertical clearance.
I'd guess the swivel would be most missed when putting an angled cut on a component which has already been worked on with the vice in its normal parallel to the table orientation. Switching to different work holding method means picking up from scratch again which can be difficult. Especially on heavily cut components where there may not be much to pick up on.
Manufacturers swivel bases are made relatively tall to provide full 360° rotation. I've often felt that a much shallower plate made to go under your conventional vice would be more than adequate for Model Engineers use. A possibly plausible approach would be to start off with a brand new, suitably inexpensive, solid disk brake rotor from a car or pick up and carefully cut the centre out. Make a washer nearly as thick as the disk a close running fit on the new centre hole then drill and counterbore for a bolt to fit your Tee nuts. Add suitably positioned tapped holes to fix your vice to it. To use fix the washer to the table with aTee nut and bolt in the usual manner and drop the disk over it. Fix the vice on and swivel to taste. Could be locked via the usual clamp bars or you could make slots on a circular arc to bolt through into Tee nuts. Maybe someone could try this and write it up for Neil if it works pout well and can be done on appropriate size machines. I can't do it 'cos I don't need it and my machinery is untypical large anyway so my methods would be wrong for smaller machine users.
My 110 mm jaw width vices are unusual in being a cross breed between normal screw vices and the precision screwless type. The nut is located by a large pin which can be passed through one of three sets of holes in the body. Hole spacing is around 60 mm, screw travel about 65 mm and overall construction is screwless vice style with an external square flange on top of the body for the moving jaw to run on. The moving jaw is some 170 mm long being, of necessity, much larger than the screwless type as it has no angled pull down screw and must rely on gib fitting like normal vices. I've been most satisfied with them and see no significant disadvantages from the hybrid style save perhaps being a little less rigid at the larger end of the operating range than a conventional machine vice. But a 180 mm capacity conventional vice is huge!
Clive