Hans, the fresh water Purasan system would be a good mate to your VacuFlush. It would be mounted within a six foot run of hose from your vacuum pump or vacuum generator. (You'll need Purasan model # PST1203). The only thing is that it wouldn't be an automatic system, you'd have to press the button on the Purasan wall panel to start the Purasan, then step on your pedal to flush the VacuFlush head. The Purasan would then do its thing for two minutes, then shut off. One thing to point out though, which may be a problem - you'll need about 2 feet of clear space about the top of the actual treatment unit, to mount the dispenser. It can be mounted higher, but not lower. there must be a minimum of 15" between the bottom of the dispenser and the top of the treatment unit. It doesn't have to be directly above the treatment unit however; it can be offset somewhat, if necessary.
Ghost, the Hold 'N Treat with a Purasan (Raritan part # 21SRP1512) would be your best choice. Same thing applies, re: 2 feet of clear space required above the top of the Purasan.