Reaper is customizable in that respect - I believe by default, though, volume automation occurs *before* any track FX. But, you can move it around in your chain.
It is - the upside I could see here would be it would give you a lot more creative range in HOW you used compression downstream, once you got the vocal track to a point where levels were more stable, and you didn't need to lean on a compressor to do that. I mostly just record myself and I write instrumental guitar music, so there isn't MUCH need for me for this, but absolutely, doing some volume automation on a vocal track can be helpful, if the dynamics are pretty variable.