I was lazy, so recently I signed up for Prime Video. It's an Amazon thingy for movies and TV shows. First I couldn't play anything, but after a short search I found that Vivaldi needs a plugin extension, with that we can play DRM content.
Luckily it's in AUR, so start a terminal and install it:
$ trizen -S vivaldi-widevine
If you had Vivaldi open, just restart it, and you can try to play. It's a sad state of internet streaming that big companies turn to DRM, and I still prefer some offline media for my entertainment.