Only 20 characters? The only Vista licence key - the PID (Product IDentification) generated from the Product Key will show up in Control Panel>System on the General tab. I'm in XP at the moment and the PID is around 23 characters plus some dashes but have look to see if Vista has fewer.
Matter of interest, why do you feel the need to know it? Is it an IBM/Lenovo thing and not a Vista licence you're looking for?