Vivaldi is based on the open source Chromium browser engine. When Chromium gets security updates, Vivaldi’s developers have to ‘backport’ those changes to Vivaldi, or leave Vivaldi users exposed to known security threats.
The Vivaldi developers do a good job of staying on top of this, and sometimes release a new version of Vivaldi in which the only changes are security fixes backported from Chromium. Vivaldi 1.15.1147.64 is the most recent example of this.
You can check your verison of Vivaldi by clicking the menu button at the top left of the browser, then selecting
About. If you’re not running the latest version, Vivaldi should offer to update itself.