You may need to wipe the HD to clean install Windows 7: Clean Install with a Upgrade Windows 7 Version
If you have files you need to copy out first, try this method: Copy & Paste - in Windows Recovery Console
If you have a Recovery partition or made Vista Recovery Disks, you could also run those to restore to Vista, but since it will set it back to factory condition you might as well get the better clean install now of Windows 7.
If you want to save your Recovery Partition, then after booting the Windows 7 DVD, select Custom Drive Tools to delete the Vista partition only, create new partition there, format before install.
If you dont' care about saving the Recovery Partition, you can wipe the HD of all code to get cleaner: SSD / HDD : Optimize for Windows Reinstallation