If you aren't worried about them being left on unnecessarily, you can just add a 30A toggle switch. That's how most vac manufacturers sell them. You don't need a separate fuse or internal breaker or relay, just don't have more than one vac on a 20A dedicated circuit.
I've set up vacs for detailers before, and they usually like them on a timer. JE Adams sells theirs with a single pushbutton and a non-accumulating timer to run preset minutes. I wouldn't use the accumulating timer if you want to limit the time on per press since people will figure out a bunch of presses will have it on for an hour.