Ras Error on 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?