What exactly is required in order to qualify for the $40 upgrade to Windows 8?

Everywhere that mentions the upgrade, at least on paper, indicates that the requirement states that you need to be running a legitimate copy of Windows XP, Vista, or 7 ^[1][2][3], and that you need to have a valid base license and one of the following operating systems. This can be interpreted as requiring that the OS to be upgraded actually be installed (which you can certainly do since you have the key).

However, Microsoft has said

"They won't need to reinstall the previous version to do the upgrade; they can just upgrade on top of the Release Preview,"

So, you should be fine. You should be able to install the upgrade on either your Windows 8 Release Preview or enter your Windows Vista key. The Upgrade Assistant will let you choose to upgrade directly or build installation media. The upgrade process^[4][5][6] will eventually prompt you for a key, and it should detect if the key is from XP, Vista, or 7 and ask you if you want to upgrade. (^{I can’t seem to find the page at the moment—I’ll link to it when I find it—but one of the upgrading to Windows 8 pages specifically mentions that the installation process detects the Windows version from the key, so it should acknowledge the attempt to upgrade.})

Worst-case scenario, you can re-install Vista with your key and upgrade to Windows 8 from there.