I did some googling of my own and found other users who located a Dell driver (R171789) for XP that can be installed in Vista using the XP-SP2 compatibility mode option. I found this driver, installed it as prescribed, went into Vista's Recording Devices, told it to show and enable all disabled devices, and boom, there was my stereo mix. So far I have been recording without any issues.
So yeah, without question it sucks that I even had to go through that, but it took me 10 minutes of research and even less than that to enable and configure.
I hope this helps somebody.