Vault report templates use an older RDLC file format. Because of the older format, newer versions of Visual Studio cannot be used to edit these files.
The biggest advantage of the old RDLC format is that there are free editors available. I tested out Visual Stuido Express 2010 and 2012, and they do not support RDLC editing. I'm not sure if there are any free third-party editors for the new format.