Like many, I have used a Vertex K4 for many years. Yes the swivel features decrease stiffness somewhat, but normally I don't work the machine that hard. Machine and vice stiffness are about on a par!.
I have tightened the moving jaw, once, to reduce the tendency to lift when tightened.
A long time ago, there was an article on making a vice alignment fixture, to save reclocking every time that the vice was repositioned.
A simplified version for my K4 is literally three pieces of one inch bar, two allen screws, two T nuts and a couple of long M8 bolts followed by a skim on both the front and back faces. It saves an awful lot of winding and knocking to and fro, and time; not to mention blood pressure!
Very early on, I bought a clamping kit, which does get used from time to time.
Also, having seen what I call "An infinite Vice" when visiting Pro Machine Tools at Barnack, ( A simple device – basically three pieces of steel set across the table of the vertical mill) I made one for my Warco Mill/Drill.
Like many things, not used very much, but invaluable when needed.
The choice of which method to use depends on what you are going to do at the time
Howard