By default, GNOME does not require credentials when using Vino for
remote access. To configure the system to require remote credentials, add or set
authentication-methods to ['vnc'] in
/etc/dconf/db/local.d/00-security-settings. For example:
[org/gnome/Vino]
authentication-methods=['vnc']
Once the settings have been added, add a lock to
/etc/dconf/db/local.d/locks/00-security-settings-lock to prevent user modification.
For example:
/org/gnome/Vino/authentication-methods
After the settings have been set, run dconf update.