Hello and welcome to the forum.
What you are describing would require and upgrade path from Vista to Windows 8. This option is not recommended with any of the available Preview versions of Windows 8 and is it not intended to be your sole OS on a production machine that you count on from day to day to get your work done.
Once Windows 8 is Generally Available and according to this chart Windows 8 upgrade paths 'leaked', XP, Vista and 7 supported - TechSpot News if you have Vista with SP1 (minimally) installed then such an upgrade path should be available to you to preserve your installed applications, assuming your hardware supports such and upgrade. There is some disagreement as to what you can upgrade from and what will be preserved during this process, since most of the information available currently is, so called leaked information. So I would be sure to check as we get closer to the October 26th GA release date to make sure nothing has changed.
Upgrade to Windows 8 Pro for $39.99