Normally, yes, you would need to reinstall Vista first.  There is a way to avoid this, however.  See this article:  Clean Install Windows 7 from Upgrade Media.  The first two methods described require only one install, but the first is a matter of luck and the second requires you to mess with the registry directly.  The third method does require you to install twice so it is almost as much trouble as installing Vista first, but you are using only the Windows 7 installation media.  If you wind up being forced to use the third method described, the key is to skip typing the Windows 7 product code on the first install.  If you forget and type the code anyway you will wind up having to start over.
Hi all, I bought my Dell Studio laptop in the period when it came with Vista and when released got the Windows 7 upgrade disk for free.I upgraded to Win 7 but now 2 years later would like to wipe the hard drive and reinstall Win7 again. My question is do I have to reinstall Vista first and then Win7 over the top again? Id really prefer not to have to do that and just install a fresh Win7 version.I would be using all the original Dell Windows Vista/Win7 disks and Product Keys. Thanks for your help.  
Re-installing Windows 7 on Dell Studio (from free Vista to Win7 Upgrade)