To autologin as a user:
"Preferences > Accounts > Login Options > Automatically log in as" and choose whichever account you want

To start all your virtual machines on login:
"Preferences > Accounts > (autologin account here) > Login Items" and add the vmx files. You'll want to make sure they can start up without prompting for input.

Switching back to the login screen while remaining logged in:
There's probably some way to do this but I don't know it off the top of my head. I say this because you can certainly Fast User Switch back to the login screen, so there's probably some way to do this in Applescript or something. I'm not sure what'll happen to the running VMs (if they get suspended, run at lower priority, run like normal...).

With Fast User Switching enabled, if you execute this script, it will take you to the fast-user login window:

[code]
/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend[/code]
Easiest way is to save it as a .tool file in your Login Startup items. I've tested this and it works.

Side-note: Your wife doesn't mind you running eight VMs in the background? Even if they were idling I would notice that!

Out of curiosity, do you need to worry about order of operations (e.g. make sure all VMs launch before switching out), or does everything get run regardless?