With the new Vivaldi 2.10 update, the browser will impersonate Google Chrome when visiting certain sites. It does this to prevent the browser from being blocked based on its user agent. A user agent is a string that is sent to a site to let it know the name and the version of the browser being used. Certain sites that detect the ‘Vivaldi’ string in the user agent will block the site and state that it is not supported. These sites will then recommend Google Chrome, which is silly as both browsers use the same HTML rendering engine.
Source: https://www.bleepingcomputer.com/news/software/vivaldi-now-impersonates-google-chrome-to-avoid-being-blocked/