Solarmass BTW I don't have Mastodon account at the moment.
I believe that as a forum member, you automatically have access to Vivaldi Social, which is powered by Mastodon. Just use the same user name and password that you use for the forum. That's what I do. The same goes for using Vivaldi's "sync" feature. In both cases, my user name is simply WetGeek.
For new users who have never created an identity for any aspect of Vivaldi, the process is very simple. When the feature (forum, sync, social, etc.) is first accessed, a dialog is displayed with a choice of "Log on" or "Register." Choosing "Register" lets you define a user name and password to use for future sessions.
I was pleased to see that Vivaldi was updated to version 5.6 with todays sync. The Mastodon icon is now on the panel to the left. At first, reading the timeline is a bit like drinking from a firehose, but the Preferences allow you to select the languages you want. It defaults to all languages, but if you select one or more, you'll see only messages written in that (or those) language (or languages).
And the "Local" tab will show you only those messages from Vivaldi Social users, whereas the "Federated" tab shows you content from Mastodon servers all over the world. If you don't select your preferred lanauage or languages in Preferences, that's where you'll be "drinking from a fire hose," so to speak.
I'm slowly learning my way around Mastodon. I don't use Facebook or Twitter, because of their policies and biases, so it felt good to discover a social medium that's without those drawbacks.