Moom actually does work with the Vivaldi browser, but only if you change its default appearance. Open Prefs, go to Appearance, check the Use Native Window box, restart Vivaldi, and Moom will work fine.

We can't make Moom work with the standard Vivaldi appearance, though, because that window's red-yellow-green buttons are invisible to Moom. Only Vivaldi can fix this, and they're aware of the problem, as they're replied in this forum thread on their site. It may help to add your voice to the discussion (in a polite way, of course)—the more "votes" an issue has, the more likely it is to get a developer's attention.