To clarify, @ThankGodItsFriday@vivaldi.net is the correct format for my username, but it doesn’t mean my posts reside on vivaldi.net. Vivaldi probably took a shortcut in adding the server name to the end of usernames because you do need a Vivaldi account in order to use their Mastodon instance. Even so, the correct domain for Vivaldi Social is social.vivaldi.net.