Flash doesn't work
It works in Chrome, but just missing from Vivaldi. For example try this link in Vivaldi and Chrome: http://www.adobe.com/software/flash/about/
Works for me.
You have version 21,0,0,197 installed
Be sure you have the PPAPI version (same as for Chrome) installed.
It seems to be a little tricky on the adobe page. Following the recommended download links (chrome is explicitly mentioned) you get only the NPAPI version. To download the PPAPI version via FlashUtil64_21_0_0_197_pepper.exe and not via FlashUtil64_21_0_0_197_Plugin.exe I needed to choose specifically a version for a different computer. Only there I got the correct version for my Vivaldi 1.0.435.26 (Developer Build) (64-bit) on Windows 10.
Thanks that worked!