@chrstos,
Vivaldi's inbuilt email client lacks the functionality to select more than one email address when composing a message from Contacts (AFAICS). Consider posting a request for this feature here, Mail, Calendar & Feeds Feature Requests | Vivaldi Forum
FYI:β when composing a message in Vivaldi's:
webmail client, select the highlighted icon shown below to right of addressable fields
2f0a95f0-d829-4169-a52e-709aa8b7191d-image.png
inbuilt browser email client, press Space-bar key to show drop list of email addresses from Contacts and scroll to locate contact or type to search. For example, typing doe shows the 2 entries in Contacts, as shown below:
5b88ee0d-155a-44da-b92e-6b5286278887-image.png