Answered my own question. After doing a little research and letting my fingers walk thru google.... it appears that ANY Vista disk will install the appropriate version that matches your product key. I had an old Vista Business CD in the back of my closet, and used it successfully to install Vista Home Premium. Yeah! So for anyone else needing to install a clean copy, give this a try! Is it legal to share an the ISO I'm going to create without giving the product key out?
I'm unsure of the policies regarding this at BC, but I've been sharing my modified Vista ISO (install any edition/architecture + OEM activation) for a few months with no issues on other communities. Technically neither qualifies as warez, but different communities draw the proverbial line in different areas, so it's always best to check. With that being said end-users should always be cautious of ISOs they find online, and should only obtain untouched ISOs, verify the hashes with those previously posted on MSDN, and then make any modifications themselves.
NO! The disk you used to install Vista home premium MUST have been a home premium disk! You cannot install one version of an OS using another Version! Vista basic, Vista Home Premium, and Vista Business or Ultimate are not interchangeable. Also, each disk has a product key, so you cannot use the same disk on different computers! You will be unable to validate a copy of an OS if it has already been validated on another computer! So, YES! It is 'illegal', but it also won't work!
The official ISO for Vista Ultimate contains the installation instructions for all editions of Vista, and can be modified to allow you to select any version of Vista to be installed during setup. The license for a OS is not tied to the media inherently. OEM discs allow you to utilize the OEM SLP key stored in BIOS, and retail discs allow you install using CoA keys - however the disc is nothing more than an avenue to install the OS, and can be used with any device capable of running Vista without having to provide activation information. Once the OS is installed, then it must be licensed, which is where the activation validation occurs. There are also scripts out there that can be added as sources to the ISO that enable you to activate the copy of Windows that is installed as long as there is a matching OEM SLP key in the BIOS.