I managed to install El Capitan (not Sierra) in Virtual Box on my MBP running Sierra. Looks fine on the first view. One major problem is that I can not sign up with my own Apple (or any other test) ID within the VM. The sign in process keeps spinning and spinning. Is it because the host system IP is already using the Apple ID? Installing Sierra into the box failed from scratch, but once the ID is up I want to try upgrading to Sierra.