For information: I also use Opera, Seamonkey (Mozilla), Firefox, Edge - No problem there
Those are using the NPAPI plugin.
You need the PPAPI one (and possibly you need to disable the NPAPI plugin from vivaldi's extension page)
1.click on https://get.adobe.com/flashplayer/otherversions/
2.Select your OS system and browser("Opera and Chromium" )*.
3.click on "Download" **
*Second step will update your Vivaldi browser
** before you download, uncheck on unwanted software!!
In Vivaldi there is as a nice function key F1, it opens the Vivaldi help website and there it is … please read article https://help.vivaldi.com/article/install-flash-player-for-vivaldi/