Boy, you know how to ask the tough questions, don't you?
Unfortunately (or fortunately, if you're an optimist like me), Vault is the price of admission for using Civil 3D. That's a direct quote from James Wedding, who I'm really hoping shows up here to discuss this one, seeing as how he's the real Vault guru. FWIW, you can find all of his nuggets of wisdom (trust me, those nuggets ARE wisdom!) at civil3d.com.
HOWEVER, with that being said......
To know whether Vault is for you or not, I'd need to know a few things. How many Civil 3D users are in your firm? What type of work do you do? Things like this will be able to help us out.
As far as calcs, spreadsheets, contracts, etc., there is absolutely no reason to put them into Vault if you don't want to. It makes for a handy storage location, but there's no requirement there. As you said, it's a document management system, no matter what anyone says.
For the design components (surfaces, alignments, etc.), I'd say that if you're going to be sharing them with anyone else on your "team", then Vault will be a good idea. Believe it or not, right now, I can see Vault working better for the smaller companies (<10 users) than the mega-40+ seat firm. There are things to beware of, things to know, and things not to do (for example, a recovered file after a drawing crash - DON'T DO IT!!!) The issue with 2007 is that data shortcuts are broken. Vault is your only real option.
I've heard two stories from trusted sources about Autodesk's stance on Vault for Civil 3D - story "a" goes something like "You wanted project management, we gave you Vault, it's what we're going to use from now on, get used to it, and make it work." Story "b" goes something like "wow, you REALLY don't like this, do you? Well, let's see what we can do...."
Make your own assumptions.