What I've done is to pass CTRL-ALT-DEL to the VM and let the screen pop up and select "change user" and manually select the user as appropriate. Yes, it's an extra step, but it serves the purpose.
I haven't used the VM long enough to know if this behavior goes away after more restarts, or if it will be the norm. I'm setting this up for my parents to be able to use their old Vista machine on their new Windows 8 machine (which they can't stand).