As a hobby/side-job sounddesign/music guy I got curious by this thread and measured it. I can't find any differences between Chrome and Vivaldi at all. So I think those who assume it must be an extension or something driver related in your case makes sense - I don't think it's the browser itself. (Makes sense, both are chromium based).
Here is my result (don't be confused if LUFS values slightly differ, especially with the song-test, it's mostly because of the screenshot time, in the moved measurement or "overall average" there was no difference. And in this case the frequency spectrum and stereo-width was the most interesting part anyway.
Pink Noise Test via YouTube:
Chrome:
chrome_pink.png
Vivaldi:
vivaldi_pink.png
Sine Wave Test via YouTube:
Chrome:
chrome_sine.png
Vivaldi:
vivaldi_sine.png
Test Song via SoundCloud (same playtime, same moment of screenshot +/- some milliseconds):
(Again: don't be confused with the LUFS difference in the moment of the screenshot, the Spectrogram is the interesting part here, if you looked at the running measurement you could see that all the other values wiggled around the same values)
Chrome:
song_chrome.png
Vivaldi:
song_vivaldi.png
Can't find any differences in audio rendering between both browsers.
Hope that it helps you, if you can at least rule out the "naked browser" as potential cause.