... the water use is small but can be adjusted on a flush by flush basis ...
They actually use 4-5 times as much water as an RV toilet.
Also, a Vacuflush system is well north of $1,000. We evaluated this system when we did our conversion because our waste tank is too far from the toilet to use a conventional RV model. We ended up going with the Microphor air-flush system instead. But both are more complicated, more expensive, and use more water than regular direct-drop RV toilets.