VCA:s or Voltage Controlled Amplifiers might be viewed at as a thing from the past. The main idea of a VCA, without getting too technical, is to control one or more faders with a single fader. This is not to be confused with a sub group though since there's no actual audio going through a VCA. You're merely controlling the amplitude of a fader with a different fader. VCA:s were a big part of old mixing consoles to be able to control faders from the listening position. VCA:s are still used on digital consoles as well but then often refereed to as DCA:s or Digitally Controlled Amplifiers. In most DAW:s however, they are still refereed to as VCA:s. One might think that there is no real use for VCA:s in the digital world when everything can be controlled with the mouse and you won't have to run from side to side at a big desk. A big mixing project can easily become hard to manage though and having a place to control the overall level of the mix and sub groups like Drums, Guitar and Vocals can come in real handy.
Therefore I tend to set up a couple of VCA:s to have more control near my master bus like this:
They work like a mini mixer to control all my groups individually and if i select all of them I can lower my entire mix going into my master. I usually end up here at the end of a mixing maybe picking up the vocals half a dB or lowering the entire mix before sending it of for mastering.
So how to set this up en Reaper? Note that I use groups for stuff as well. So I have my VOX, DRM, GTR and BASS etc. set up like normal busses or folders as well:
The busses are then added to a VCA follow group that you can get to using SHIFT+G:
The VCAt racks are set to VCA-Leads using the same menu. Note that there's only one type of track in Reaper and you make these do different things depending on the properties. With everything assigned I now control the VCA follows (sub groups) with the VCA Leads.
I want to have it set up like this to maintain gain structure into sub group compressors and limiters.
Don't forget to assign a VCA to your return tracks as well. You don't want to mess up your reverb sends when you lower your entire mix!