Open terminal and type:
If Google Chrome is not installed on your system, to get Vivaldi browser to use the Google Chrome Pepper Flash plugin, you can install a package called "pepperflashplugin-nonfree".
sudo apt-get install pepperflashplugin-nonfree
Then restart Vivaldi. That's it!
Important note: since Google Chrome is now only available for 64bit on Linux, it means that its Pepper Flash plugin is also only available on 64bit. So the instructions below only work on 64bit! Flash might work on Vivaldi 32bit if you have an old Google Chrome for 32bit installed, but it won't receive any updates so you shouldn't use it.
Vivaldi supports the Google Chrome built-in Pepper Flash plugin, but it doesn't come bundled with it. If you have Google Chrome installed on your system, Vivaldi should already be using its Pepper Flash plugin, so there's nothing you need to do.
Check Flash plugin shows PPAPI setting after install by visiting vivaldi://plugins