Be well informed and try to do reverse engineering with Chrome. Chrome, just like Vivaldi, is based on Chromium, but from there the similarities end. You don’t need to reverse engineer Vivaldi to modify the proprietary UI, the only part that is. You can degoogle Vivaldi yourself in the privacy settings, since, unlike others, in Vivaldi it is an option, because there are users of Google services (students, professionals…), which require certain Google APIs to work, even Firefox incorporates them because of this and also ‘ungoogled’ Chromium, but where you can’t disable them, in Vivaldi you can, if you don’t want to use Google services (Crypto Tokens, WebstoreAPi if you want to use extensions from there, and some others), the internal Google telemetry APIs are already removed from Vivaldi by default, same as in a degoogled Chromium.
That is, you can degoogle Vivaldi as an option to such an extent that Google doesn’t recognize it as Chromium, if you want. But in this case some pages and services will not work for you, If a “degoogled” Chromium can use Gmail, Blogger, Hangouts or now Google Chats and extensions from the Chrome Store, it isn’t really degoogled. Even Firefox, less the Google Store, can use these, a degoogled Vivaldi with all APIs desactivated, can’t.
Be well informed and try to do reverse engineering with Chrome. Chrome, just like Vivaldi, is based on Chromium, but from there the similarities end. You don’t need to reverse engineer Vivaldi to modify the proprietary UI, the only part that is. You can degoogle Vivaldi yourself in the privacy settings, since, unlike others, in Vivaldi it is an option, because there are users of Google services (students, professionals…), which require certain Google APIs to work, even Firefox incorporates them because of this and also ‘ungoogled’ Chromium, but where you can’t disable them, in Vivaldi you can, if you don’t want to use Google services (Crypto Tokens, WebstoreAPi if you want to use extensions from there, and some others), the internal Google telemetry APIs are already removed from Vivaldi by default, same as in a degoogled Chromium. That is, you can degoogle Vivaldi as an option to such an extent that Google doesn’t recognize it as Chromium, if you want. But in this case some pages and services will not work for you, If a “degoogled” Chromium can use Gmail, Blogger, Hangouts or now Google Chats and extensions from the Chrome Store, it isn’t really degoogled. Even Firefox, less the Google Store, can use these, a degoogled Vivaldi with all APIs desactivated, can’t.