I use SourceGear Vault and generally love it. Guessing the following is not directly related to Vault but rather source control in general. By default, Vault is configured to ignore the Bin folder within an ASP.NET project or Web Site.

What have I been doing for years when I need to import a vendor's DLL to my project? Yup, I copy it to my Bin folder and add a reference to it. Wrong thing to do!

That 3rd party DLL is not achieved along with the rest of my project since Vault is configured to ignore the Bin folder. (Guessing that all source control systems follow this behavior.) What should be doing instead? Create a folder within my project along the lines of AcmeLibrary and copy DLLs there. Then reference my DLLs and Visual Studio will copy them to your Bin folder when you build your project. The AcmeLibrary and its contents will be placed under source control.

Hope this saves you some future grief!