Well good news, if you want to get a new scanner, you can use this as an excuse, but you can get the Visioneer 5800 working under Vista. There's a technical note from Microsoft support site on this.

Visioneer 5800 USB Scanner - Make sure to put Windows XP SP2 compatibility on the 5800 driver install program before installing the driver and restart your computer after you get it installed successfully.

Basically you lie to the driver installer by setting compatibility mode back to XP SP2, Here are instructions... http://www.lockergnome.com/nexus/windows/2007/04/09/run-applications-in-compatibility-mode-in-vista/

You want to do this on the scanner installer executable, you can either download the file again, or if you still have the XP driver CD you can do it there.