welcome to the forum!
Vivaldi Mail has next to no contact management as of yet so the answer is: you can't connect an ldap address book. The contacts list is filled by adding email addresses and names from the information in the email database, i.e. when you have an email from Some Person [email protected] it will add that to the contact list you find in the panel. You can then make some edits there but it's really quite basic as of now.
Vivaldi can import Contacts from Thunderbird - I have never tried this myself, I just see that the option exists in the Import&Export function. Maybe you can import LDAP contacts to Thunderbird and then import them to Vivaldi. Otherwise - LDAP has hardly been discussed on the forum yet.
You may be interested in the feature wish for CardDav sync https://forum.vivaldi.net/topic/53314/contacts-carddav-sync