Plays as html5 when chrome://flags/#prefer-html-over-flash is enabled or
flash with prefer-html-over-flash disabled(default).
Ubuntu 16.04 with adobe-flashplugin installed.
Something I have noticed is, when I don't have the adobe-flashplugin package installed, Vivaldi will pick up google-chrome's PepperFlash from ~/.config/google-chrome/PepperFlash/
Being a user directory, it is not removed when you uninstall google-chrome so could leave behind an old flash version for Vivaldi to pick up when flash is not installed.
chears! all sorted now
[i had installed Chrome to play Netflix on Vivaldi and it had also inserted Flash into Vivaldi without me knowing]