how to install windows 7 64 over windows vista 32

There is no upgrade path from 32bit to 64bit, you can best do a scratch Win7 installation and use the Easy Transfer Wizard to bring your files from Vista to Win7.

Caution: If you'll be using the same HDD for this, you'll need to collect your Easy Transfer Files first and write it to some external media or USB Flash Drive, before you write over the Vista operating system.