Be aware that the control path is global, and there is essentially only one "port/channel" for it. That means that every CC you try to map to anything must be exclusive or else it will do multiple things at once. A number of actions are mapped by default to control certain things, and it can be really annoying when, for example, you go to tweak an effect parameter and end up scrubbing the playhead or panning track 7 at the same time. I only use it for actual transport and mixer tasks.
For VST parameters I prefer
CC to Parameter because it's far more discreet, and allows you to use all of the different routing features of Reaper.