By default, Vivaldi uses its own New Tab page. It also doesn’t allow changing of the New Tab page by extensions by default. Because of that, you have to manually enable customization of the page.

To enable Flowtide after installation, open vivaldi:settings, navigate to the Tabs section, and under Start Page > New Tab Page, toggle on “Controlled by Extension.”

After these steps, you should be good to go! Enjoy your new New Tab page!