rasdial error 64 bit vista

I am attempting to establish a dial up connection using RasDial. The code works without any error on all operating systems except for the Windows Vista 64 bit build.
After calling RasDial, I am receiving an error during authentication. The RasDialFunc callback function is called once with RASCS_Authenticate with no error code, and then called a second time with the error code 0x80070057. This matches a standard COM error code: E_INVALIDARG.
The fact that the same code works under Vista 32 bit, leads me to believe it has something to do with the differences between the 32 bit build and the 64 bit build. Any ideas?