First, I want to talk about this blurb on @Vivaldi's sign-up page.
It's clear to me that Vivaldi is using Mastodon as part of a suite of social tools that include:
1. Webmail account
2. Mastodon account
3. Blog hosting account
This is part of how they aim to differentiate themselves from Google Chrome.
Sidenote: They're also asking you to give them your phone number.