Vivaldi is an ungoogled Chromium, there don't go any data to Google, except if you use the optional Google Save Search in the privacy settings. OpenSource, well, Vivaldi isn't strict OpenSource, because 5% of the script corresponding to the UI is proprietary, but full auditable and even accessible and moddeable by the user, in the forum they show how to do it (logically at own risk). There isn't any privacy issue or hidden things in it. User data in a Mozilla-Firefox Account is shared with Alphabet, googleanalytics and google-tagmanager, in Vivaldi nothing is shared with Google or other companies.