They get paid to set the default search engine to Google.
Vivaldi generates revenue from partner deals with search engines. Every time you search using one of the pre-installed search engines, you’re helping us grow, one search at a time. Currently, we work with DuckDuckGo, Ecosia, Startpage, Yahoo!, Bing, and Yandex. The only exception is Google – we don’t make money when you search with Google. However, we know that some of you use this search engine daily, so we include it in Vivaldi.
(Source: https://vivaldi.com/blog/vivaldi-business-model/)
The other other source of revenue is bundled partner bookmarks (see again the linked document).
Not affiliated in any way, just thought if someone wanted to, they might donate.