hi,
reinstalling vista is very easy:
just follow the wizard, then you are done with it.
No reformatting is required at all.
Here is a simple guide:
Easy reinstall for Windows… - Cyberwalker.com
However, for safety reasons(virus, corrupted data file, hdd bad sector etc), you may want to firstly format your Vista root drive (for example, C:\).
To do this, you should find the disk preparation page of vista's installing wizard. With a single click, you can have your root drive formatted.
hope this helps
Dex