Even Vivaldi will add in the future a unique id to the browser in order to track what users do while on line, though it should have an option to deactivate it. This info is taken from Vivaldi dev blog.
"We do not collect any usage data."
"...every 24 hours, your browser, if running, will send via https, the following info:
- The unique user ID
- Browser user agent
- CPU architecture
- Screen Resolution
- Accept Language "
So it's nothing serious.