It doesn’t really matter what kind of block it is. Vivaldi comes with TLS (Transport Layer Security overlay), but it is not necessarily turned on if its an old install. If a fresh profile has it turned on, then all traffic leaving the browser are encrypted and cannot be tampered with. you can check if it is turned on–

go to Vivaldi://flags

Vivaldi has TLS active by default at some point for new installs. If new profiles also have that then that’s what is happening.

The browser is not using the ISP’s or the computer’s DNS settings. So DNS blocking/redirecting won’t work on it.

there’s the ole https://www.reddit.com/r/darknetplan/

kitschy name, but when it was established it was not even planning anything like what it is doing now. meshnet is the section you’re looking for.