hi,
you have to copy some files from the 2008 server to your vista client. if you use a win32 vista you have to copy these files from a server2008 x86, if you use x64 os the server must be on x64 too.
copy the following files from the server (here its a 32bit os) %systemroot%/system32:
mprsnap.dll, rasuser.dll and rtrfiltr.dll to the vista system32 folder.
now copy the following files from the server (here its a 32bit german os) %systemroot%/system32/de-DE (en-EN when using english os):
mprsnap.dll.mui, rasuser.dll.mui, rtrfiltr.dll.mui to the vista system32/(your-language) folder.
now start a cmd using runas admin on the vista client and enter: "regsvr32.exe rasuser.dll"
re-open your ADUC console... there should be now a dial-in tab.
btw, this workaround is not officially supported but it works ;-)