A note found while floundering randomly looking for help (mostly unsuccesfully) at Microsoft notes that if you want to multi-boot to Vista and to an older Windows version, the OLDEST version must be installed first, and successive versions added in order.
It appears that the older version can't be opened after the machine startup that happens before Windows is loaded if that "startup" comes from a Vista boot sector(?), but if the machine turns on with the older version, Vista can "update" the machine startup when Vista is selected as the one to open.
The article was "vague" and wasn't particularly relevant to what I was looking for, so if my comment doesn't make much sense just ignore.
There was no comment (that I noticed) on requirements to boot a non-Windows OS on a multi-boot machine with Vista on it.