The LDAP interface can be activated within the VIMP backend at Configuration -> Users -> Authentication provider.
In case the LDAP module is installed, you can select "LDAP" instead of "Framework" (default).
Important: As soon as you switch to "LDAP" you can only log in via LDAP, no longer via the internal VIMP authentication! If your LDAP settings don't work and you want to switch back to "Framework" authentication, follow the steps in this article.