Since it has 3 dvd(s) you will be fine. usually the first dvd is the regular o/s install DVD and the others are the rest of the image.
You will be able to do as you want a full restore to Vista whenever you want. You can wipeout the partitions also,
However, Windows 7's clean install needs to have a Windows O/S on it already in order to a clean install using the win 7 upgrade dvd. There are ways to get around that,also. Either way, you will be fine. You would always be able to get back to Vista as long as you keep your Vista Recovery DVD set.