added gui, but cannot log to gui as original user

I added xorg, openbox and lxqt, and latter mate to my ubuntu 18.04 vagrant box to create lighweight gui experience. After restarting box, VirtualBox window popped up with invite to select a user and enter a password but there is no user to choose (by default all vagrant boxes have a vagrant user)

I can add another user, but would prefer to use the 'vagrant' user, as some software is installed in his home dir /home/vagrant

How to log in as vagrant via GUI?