@Mattonius,
As i know, Vivaldi is using chromium kernel, so cleaning Vivaldi should be similar as Google Chrome.
I created a UserCustom.ini for wise disk cleaner, it can clean Vivaldi and Yandex browser. I just begin to use Vivaldi, I'm not sure this file can work perfectly.
When you download it, please change it to UserCustom.ini, then paste it to WDC folder.
UserCustom.txt