A lot of updates ago, Vivaldi "hijacked" those Plasma settings. If you want to have the buttons on the left side, you have two options:
Use Vivaldi native settings. Go to Settings > Appearance, and in Window Controls Position, select Left Side. You will not be able to choose the order of the buttons, though.
Enable the KDE Plasma native window. Go to Settings > Appearance, and mark Use Native Window. Restart Vivaldi. Once you have the Plasma native window on, you can go to KDE Plasma native System Settings > Appearance > Window Decorations > Titlebar Buttons bar, and set the buttons to your taste.
If you didn't understand some instruction, please let me know and I can show you screenshots.