First off, I'm using a Peavey Pi100r or something like that, a 2i4, and Reaper. Now I know the mic's not the best but I don't think it's the issue. When I record vocals I changed my volume here and there. But I can't seem to find a good input gain. The louder vocals distort and yo can't hear the softer vocals.
Fender Telecaster w/ Dual Gibson Humbuckers
Ovation Celebrity Acoustic/Electric
Fender Frontman 212R
Old Too Young (Folk/Bluegrass/Punk)
The Orange Line (Stoner Metal/Punk/Alternative)
That's what compression's for (or gain riding, but compression's easier). Record at a level where nothing distorts, then once it's recorded you can add compression in post to bring up the softer parts.

If it's a problem for tracking then if you use the DAW's monitoring rather than the 2i4's direct monitoring you could add a hard compressor/limiter for tracking, then remove it once you're done and edit in post.
If the dynamic shifts of vocals are between sections. I.e Verse/Chorus. You could just record a verse... reset gain.. chorus. And so on.