That’s incorrect. social.vivaldi.net uses your vivaldi.net account details for the account. It’s detailed on the sign up page: login.vivaldi.net/profile/id/signup
Emphasis added.
With a Vivaldi account, you can synchronize your Vivaldi browser data, participate in forum discussions, connect with people on an open social network (@YourUsername), and host a blog on Vivaldi.net (YourUsername.vivaldi.net).
New to the Fediverse and Mastodon? Get onboard with Vivaldi Social, the new ad-free social network | Vivaldi Browser (vivaldi.com)