Ever since I put Vista on a Dell Latitude D810, I've had issues recording speaker audio on that machine. I didn't think much of it, since I used to use another XP machine as my main machine anyway, but it recently came to a head when I got rid of my XP machine and needed to actually record webinars on a Dell Precision M6300 with Vista on it, and I once again was painfully reminded that it doesn't work.
First I thought it was Camtasia's fault, but not at all. It's Dell that is disabling the functionality in their Vista drivers.
After hours of searching for a solution, I gave up and just used a cable ($5) between the head phone and microphone jacks. Worked great, except for the fact that you can't monitor what you're recording (unless you feed it through a head phone splitter or mixer).
Several nights later I was determined to find a solution for Vista, because I grew to hate the ridiculous cable, and after hours and hours of digging through web sites that made McAfee go insane :p I found the solution!
Install the XP driver on the Vista box in XP compatibility mode. Works like a charm!
Simply save the XP audio driver (in my case Dell's XP driver for Sigmatel) on the desktop, right click on the file and go to the Compatibility tab and select XP Service Pack 2 for instance. Then install the driver. In my case the installer would fail on Vista, but it works just fine in XP compatibility mode.
After the driver is installed you may have to go in and actually show the disabled devices and enable Stereo Mix under the Recording tab in Sound properties.
If this helped you, please feel free to leave a comment saying so. :)