Theog's approach to unplug Vista drive during install is best. It will later allow Vista to be booted via BIOS shortcut Boot Menu key (usually F10 or F12) if you want to boot it instead of Windows 7 which will be set as first HD to boot.
At any point you can deactivate Vista drive by booting the Windows 7 DVD, selecting Repair, clicking through to Recovery Tools list to open a Command Prompt and type:
SELECT DISK # (replace # with Vista HD)
This will allow you to delete it in Disk management after moving data off it you want to clean off the MBR, or just delete the Vista system files.
Use one of the methods here to install Upgrade version to clean HD: Clean Install with a Upgrade Windows 7 Version