I have already blogged about Vault and source code control here: http://www.dualbotic.com/DasBlog/What+Visual+Source+Safe+Doesnt+Do+For+Us.aspx
We are a seven man MS dev team and have been using Vault for nearly a year. First things first it is a huge improvement on VSS 6.0d, because it uses compression combined with a webservice our dev's can work from home over broadband hardly noticing any slow down in check ins/outs. The other killer feature is the Diff tool this is big step up from VSS. It also has some neat reporting although it takes a while to figure this out as the UI is not designed all that well. The one bug bear we have is that there is no plug in for VS2002 only for VS2003 over time this will cease to be a problem as we move more of our code base to VS2003.
I haven't looked at the newest MS VSS which ships with VS2005 Team System so I am not sure whether Vault has some competition coming it's way but we will probably be using VS2003 for at least the next couple of years so we are very pleased with the utility we are getting from it.
One thing I have noticed with all source control products is that the less experienced members of the Dev team struggle with the concepts that lie behind source control and that the UI of most tools doesn't do much to assist them in their plight. It would be good to see more narrative in the UI that would help them make the right decisions and not overwrite local changes by mistake.