If I have a VMS10 and want to buy the upgrade to VMS12, can I then still use the VMS10 on another computer or the "upgrade" means the same serial number is used from the VMS10, hence rendering it unlicensed if used on other machines ?
Yes, no worries. All products have their own S/N's and you can have both versions installed on your computer running independently if you so wish. What's on 1 computer has no bearing upon what's installed on the other so if you wanted v12 also on the other computer there's nothing to stop you.
By the way, by clicking on 'My Account' above you can always check what products you have bought and their associated S/N's.