I have also driven vacuum and air over here in GB. Juctionmad if you are asking, I apply the brake and leave the handle at one position what will happen? with either system you will stop. The vacuum will continue to increasingly leak on and apply the brake harder on the train. The things to remember with vacuum are, the brakes on the front of the train apply first but release first so the rear will still be braking when the front is released. So a gentle initial application then quite heavy to get the train under control and then a gentle, apply release technique to slow to a stop. The plan is always to stop on a rising brake e.g. heading towards release and a 15" application at maybe just rolling to come to a stand. Hold the train on the air/steam brake and blow the train brakes to release ready to go.
On BR 21" of vac was the norm and brakes really start to bite at 15" of vac. Slowness of release is mainly due to the vac; cylinder relies mainly on its own weight to fall and release but they are bloody heavy! and all the levers, fulcrums and rods used in the system. Of course all this depends on the quality of the braking on the carriage set you are pulling, no two will be the same so lots of variables for your driver to suss! When I was on BR steam I fired regularly to a driver who on stopping passenger trains had two brake positions when running into a station; full application and release with the large ejector wide open. Very rapid stops! I would have the blower on,a white hot fire and the injector going filling the boiler. The departure would be just as rapid. Western 6100 tanks could really take a lot of abuse! Hope that lot helps your questions and sorry for the wave of nostalgia.