A byproduct of heavily distorted electric guitar is compression. Therefore, the level of the guitar is, in all likelihood, going to be pretty consistent across the recording. Anything in the same general frequency span is going to be lost if its level is below it. Vocals are the main candidate in a rock/metal tune, so if they’re very dynamic, it could be a problem.
You'll have to take the singer's range and the guitar tone into consideration. If the guitar has that super heavy "scooped mid" tone, it may well leave plenty of room for the vocals. If it has more midrange, you'll have to be more careful.
In this particular case, I would record the vocals as hot as possible. That gives you more to work with. I would also experiment with a little analog compression before the AD conversion, if I had that sort of equipment at my disposal, just to keep things tame. But do try to get the strongest recording you can without clipping.
A lot will depend on the singer. If the singer has decent mic technique, understands how to use proximity effect, backs away for the screams, etc., you’ll have a much easier time.
Every singer, microphone, and mix is different and there is no magic number. You’ll probably have to apply at least some compression to the vocals to get them to be articulate and present. If there are whole quiet sections, move them to another track and handle them differently, or use envelopes to control parameters on the compressor, using care to make this as transparent as possible. (Or you may not need to, since the whispers will be different enough from the screams, anyway.) Most importantly, use your ears and good judgement. If it sounds good, it is good.