Yes, you can dual boot Windows 7 with Vista. You can even do it with an Upgrade, although you are supposed to stop using Vista once the Windows 7 Upgrade is installed.
Since you want to keep your Vista, I would buy a full retain version of Windows 7, boot from the installer and install it to another HDD.
If you want Windows 7 to configure a dual boot menu, leave the Vista drive plugged in along with the Windows 7 target drive. But unplug all other drives during install.
If you unplug the Vista drive during Windows 7 install to another drive, you can control which drive it boots to by setting that drive to boot first in BIOS, or using BIOS boot order shortcut key (normally F10).
The drives should still be visible to each other once both are plugged in.