Hello xxsomethingxx,
No, you do not need to purchase another copy of Vista. You can use your same product key number from your 64 bit Vista Home Premium to install the 32 bit version of Home Premium from a Vista 32 bit installation DVD that is the same type (retail or OEM) as your 64 bit DVD.
This link below will help you determine if something is compatible or not.
Windows Vista Compatibility Center
Hope this helps,
Shawn