The current Vista extended kernel was abandoned as a buggy mess and is closed source, has a messy installer, and generally isn't that great.
There are options like One-Core which use open source projects (ex. ReactOs) and COULD be ported to Vista,
You could even try and use Wine to make a VxKex-esque system.
Why has nobody done this? Vista has the worst extended kernel out of the XP-7 range (its just above Windows 8.1 which has none)