I never had this problem to put Vivaldi or any other browser instead of EDGE. Maybe i’m still with W10, maybe the last Windows which can be tamed. The only thing is that in Windows it isn’t enough to declare the browser as default in the browser settings, you must do it also in the Windows default app settings too, but after this you’ll never seen EDGE again, at least in W10.
I never had this problem to put Vivaldi or any other browser instead of EDGE. Maybe i’m still with W10, maybe the last Windows which can be tamed. The only thing is that in Windows it isn’t enough to declare the browser as default in the browser settings, you must do it also in the Windows default app settings too, but after this you’ll never seen EDGE again, at least in W10.