If this is factory-installed Vista, then make your Recovery Disks so you have a path back to Vista.
You also have the option to keep your Recovery partition (from which the disks are made) which can itself be booted to run to restore to factory condition. If you want to keep the factory Recovery partition, then don't wipe the HD.
However, wiping the HD is the cleanest possible install since it overwrites all the old factory code, corrupt code, even infected code with zeroes so nothing survives. Just deleting partitions and'or formatting erases nothing.
A middle ground would be keeping your Recovery partition but wiping just the C partition for install. This partition-specific wipe can be done using free Partition Wizard bootable CD. See option 3 here: Partition Wizard : Use the Bootable CD
However since you probably won't want to go back to Vista, just having the Recovery disks on hand should be enough to restore factory Vista in case you want to sell the machine but keep your Windows 7 to use elsewhere. And with built-in backup Imaging, you will have a method to reimage your HD with Windows 7 easily in 20 minutes if Windows 7 ever becomes irreparable. If there is anything specific you don't understand after reading the steps I gave and the links provided, please ask back your questions and I will gladly answer them, if you are interested in learning.