Do I need Vault?

You might. I am not saying that you don't.

See www.civil3d.com and read James Wedding's blog, check out www.autodesk.com/civil3d, watch webcasts, talk to your reseller.

But my general thoughts are:

If you are just learning Civil 3D and trying to see how it fits in with your firm, you probably don't right now. In the beginning (depending on how your firm works, or is willing to try working) you can get away with a few other ways of sharing data, or perhaps keeping all of the modeling data in the same drawing, or using LandXML.

Vault is certainly a superior way of sharing data for large firms where they really need to share project data (vs. just XREFs or DWG data), but it takes a lot to get Vault working like a swiss watch, and if you are still learning how to make alignments, maybe you can wait before you learn to share them.

I will elaborate more on what Vault does later, but here are some images that might help get you thinking.