Add the effect to your effects bin.
Right click in the track area
Add envelopes, track envelopes...select your plug-in.
It will then ask what control on the plug-in you want to create the envelope for. After that...adjust on/off/more/less as you like. You can do this manually but editing the envelope with your mouse by adding adjustment points (click on the line you'll see what I mean) or, if you have a controller, you could arm the effects for automation then playback and adjust as you listen.
Another option would be to add a send on the vocals track...send it to an effects buss. Same idea just outside the actual track.
Good luck...I'm at work right now so I can't do any screen captures or detailed step by steps but you'll get the idea.