If you have enough space you can create a separate partition and install Windows 8 upgrade to the secondary partition. Unlike previous versions of Windows, Windows 8 upgrade Pro allows you to install it as a full version to a secondary partition after it detects a previous version of Windows from XP forward.
That way you will create a dual booting system and can ensure you can still use the older programs without being forced to upgrade to newer versions. This will also come in handy for devices that may not be compatible with Windows 8.
If later on you decide you don't care for Windows 8 you can simply remove the partition and repair the partition to simply load Windows Vista only.
If you decide to get rid of Vista instead you can do that too and possibly resize your partition table to either grant Windows 8 more space or create a partition for file storage. That depends on how your partition table is currently setup.
The other workaround would be to delete the Vista partition and if there is enough space you can clone the Windows 8 partition with special utilities, delete the original Windows 8 partition and resize the main partition to use up all the drive space.
You will need experience to pull this off but just making you aware of the options.
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If you intend a clean install contact Microsoft first and check to see if they are willing to reset your Microsoft Office key. Tell them you are upgrading to Windows 8 and just want to make sure re installing it won't be an issue.
I've had keys reset by Microsoft before. You just need to prove you are the licensed with either a receipt directly from Microsoft or the name the keys are registered to.