How to handle extreme dynamic range in a vocalist?
What Kim Burgaard said is very true. The vocalist has a lot of control over how their voice is recorded. As I'm a much inferior vocalist to those I record, it can be difficult teaching them this technique, as you just want to get a good take.

So what I end up doing is have the vocalist scream. Well, I ask them to warm up and hit the loudest levels they can. I then adjust via that. I try not affect overall sound with compression in the analog domain. With the takes in 24bit, I can use envelopes or a compressor in the mixing stage.

I would stay away from ridding the level during recording. It's more difficult to get the levels right in the mixing stage.

You mention running a second mic for the vocal. I would hope that is what you are doing anyway. I don't think recording the mic and guitar with one mic is a good idea at all. You'll not be able to mix anything.