[2.5.1525.37] Bug: Completely black window, but menus do render correctly
https://forum.vivaldi.net/topic/37218/

Can you please start Vivaldi with a special desktop shortcut, change its target to
"C:\Program Files\Vivaldi\Application\vivaldi.exe" --disable-gpu
That makes see again the content.
Open Vivaldi Settings -> Webpages and Disable Use of Hardware Acceleration

とりあえずやってみなよ