Have you noticed substantial differences in website rendering on IE 6 or IE 7 when run on Windows XP versus when run on Windows Vista?
The browser engines should render the same, however it is unlikely that there is an instance of IE6 installed on a vista box that is not run through a VM or VirtualPC.
The rendering is the same as far as I've noticed, as long as you compare apples to apples (meaning: IE7 looks the same on both, but IE6 may not look the same as IE7). Vista might do some extra security enforcement, but I'm not aware of any specifics. And there's no such thing as IE6 on Vista.
We have recently done IE7 testing for both XP and Vista. To my recollection there are no noticeable differences for IE7 on XP vs Vista. We did not attempt to install IE6 on the Vista machine.
There are differences between IE6 and IE7 on XP though. I won't go into detail as I'm sure you can easily find hundreds of threads by doing a quick good search. Our biggest issues were some JavaScript problems when integrating with ASP .Net web applications.