I have built this sample project, and it runs fine, but I'm having limited success with the debugger. The instructions say...
To enable debugging of Fortran code called from Visual Basic, right click on project "VBCallsFortran" and then select Properties. In the left column, select Configuration Properties > Debugging. Check the box labeled "Enable unmanaged code debugging"
VS 2012 doesn't have that option, but there is Enable Native Code Debugging. Checking that gets the debugger to step into and through the fortran code, but the values of arguments cannot be examined, although local variables can. Is there something else I need to do to enable this?