The problem is that I start the VM with GUI have installed with ansible lightdm-gtk-greeter-settings,
but I'm asked for ubuntu's user password instead of the vagrant user.
This is the problem I'm trying to solve.
It seems the vagrant user is not added to sudoers on the official Vagrant ubuntu/trusty boxhttps://atlas.hashicorp.com/ubuntu/boxes/trusty64
The fix was to add these lines as inline shell provisioning: