There has been a hack and an official KB on how to run ADUC under Vista. And while not perfect, it is functional. No one I know can understand Microsoft's logic for not having this solved prior to shipping Vista.
If you also want to enable the Exchange extensions in ADUC there is one more step. After you do the things mentioned here, copy these three DLL's from a Windows 2003 Server (c:\%systemroot%\system32):
- netui0.dll
- netui1.dll
- netui2.dll
Then re-register regsvr32 \windows\system32\maildsmx.dll.