I'm assuming you are using IDE integration. Correct me if I'm wrong.
Are you trying to add a solution to source control, or are you trying to open from source control?
What version of Visual Studio are you using and what version of Vault?
Can you confirm first that you can open a Vault GUI, login, and access the files there? If not, can you tell me the errors you get there?
Access failure
Moderator: SourceGear