Just to illustrate, here are screenshots, first from the machine that works, second from the one that doesn't.
As you can see, no objections on WME9's part.
Whoops, uh oh, now apparently I can't have a stream over 20mbit.
I found a Knowledge Base post about a hotfix allowing CBR videos over 20mbit in the command line, but nothing about in the UI or to do with VC-1 specifically.