Big Vivaldi Mail update with prefetching, previews, and quick replies. Vivaldi 6.8 also includes real-time tab memory usage, improved address field, and more.
Assuming manifest v3, it shouldn't break. However, some extensions will stop working. Vivaldi doesn’t have its own extension repository so they will be phased out there just like in Chrome.
As far as I know Vivaldi is quite prepared to get things working as much as possible with Manifest V3 and putting a lot of work in their own adblocker as well because of it.
There is a version of UBlock Origin that works with V3 but it is limited compared to normal one, will see how it goes but for now I think they can delay with the V3 update for some time.