I keep Vista on my Win7 USB install stick which is also my tool kit with several dozen installers on it. When I need to install Vista, I sweep the Win7 install files into their folder for storage and spill the Vista files out into the root of the stick.
The stick was originally set up by writing Win7 installation ISO to it using Universal USB Installer
with WIn7 in the dropdown menu. I then copied in the other tools and Vista extracted ISO in its folder.
As stated whichever OS you're needing to install must have its files spilled out into the root to boot.
Why would you want to take Win7's branding off of the installation process?