Vivaldi workarounds these kind of issues by identifying itself as chrome. When a site blocks vivaldi because it is not chrome the site is added to a list and it will identify itself as chrome in the future (I know because it happened with the web version of whatsapp).
>>jorgem+(OP)
It's actually a whitelist. Vivaldi always uses the Chrome user agent, unless it's a website known to not block it, like vivaldi.com, and probably some of their partners too.