I tried on a fresh image and v6.5 runs, that's great. Since this year, current releases of the chromium engine do not work on Windows 8.1 anymore, so this will happen to buster one day as well. But not yet
This is the error I get when launching vivaldi from the terminal:
[3650:3650:0211/192259.919647:ERROR:gpu_process_host.cc(994)] GPU process exited unexpectedly: exit_code=11
[3650:3650:0211/192259.919713:FATAL:gpu_data_manager_impl_private.cc(448)] GPU process isn't usable. Goodbye.
A quick google showed that this error emerged with recent chromium versions on some configurations. So this may get fixed in future releases.
[Edit: ok, writing took me much too long, good to see that the current Vivaldi does work after all]
Bummer that you cannot use an up-to-date version of Vivaldi
Well, at least there is some version of Vivaldi available
With the Pandora this really became a reason for me to not use it for mails and browsing any longer, though. (Plus I got other devices which performed much better at some point).
I think instead of deleting the repository you can also do something like:
apt-mark hold vivaldi
That would enable you to unhold the package in case for some reason an update becomes possible later.