Why does it have to be written in .NET ? I understand why you wouldn't want it in something like Java, but C++ should be OK? I've used TFS, VSS, SVN, and CVS and I like SVN the best (apart from the lack of VS integration but you get used to that). Heard good things about Vault.
I'm currently debating TFS (Orcas) vs SVN for my project, I like the fact everything is out the box and integrated of TFS (once its installed) but SVN is lightweight and easy to use...
If it happend to be opensource, it would be really nice if it were written in .NET C++ would work, but im not any good in C++.
I just tried installing Vault after finding out that its free for a single person. After a few hours of pain trying to install it, it seems to work. I like it.