Both tools produce XML, but the Report Builder is part of SQL Server Reporting Services, and assuming that you are on SQL 2008, adheres to a different schema. The Visual Studio report editor generates SQL Server 2005 reports. They are not interchangeable and cannot be converted (at least I am not aware of a tool that can successfully handle the conversion).
The Report Viewer control (for client rendering) can only handle SQL Server 2005 report files (same as Visual Studio 2008). Reporting Services 2008 can only handle SQL Server 2008 report files.
This issue has been discussed here.