"My biggest problem is that I like the raw vocals and I hate them when they are mixed: in the raw, I sound like me, with a lot of dynamic range and tone color -- there is a vibrance and "edge" to the vocals that I like. When the tracks are mixed, I sound like a Barbie doll and all of the excitement has been blah-ed out of the performance."

I would say that that is as clear as a bell as to what the problem is.

The guy mixing it should understand exactly what you mean from that - he in turn should have the technical ability to make it NOT sound like that. I wouldn't have thought that you have to say to him 'I think you have over compressed the vocal and you need to boost the low mids blah blah blah'. That's his job!

I went to a studio with a friend and played a bit of bass on a couple of tracks and found it very hard to answer the question that the engineer asked me when he said 'and how exactly do you want the bass to sound?' because I'd never really thought about it! (I think 'nice' was what I really wanted :)!!) So I left him to it.

You, however, seem to know EXACTLY what you are trying to achieve - ie what you described above - and he should have the tools to do it.