Hello,
at least I can confirm, that NS7 works very well as AD in my installation. I did not try to run NS7 on Virtual Box, but running on Proxmox.
What might be important to join the domain on the Windows machine is the registry patch as noted in the docs (win7samba.reg). Without it you might fail to join the domain - for me it was mandatory (W2012).
Also here: http://community.nethserver.org/t/install-nethserver-as-ad-with-linux-and-windows-clients/866