I just resolved this after much, much googling:
the bug is in xdg-desktop-portal-gnome. I did a pacman -Q | grep xdg-desktop-portal and found 3 packages installed:
xdg-desktop-portal
xdg-desktop-portal-gnome
xdg-desktop-portal-gtk
I uninstalled xdg-desktop-gnome and restarted the xdg-desktop-portal user service:
systemctl --user restart xdg-desktop-portal
I closed and restarted Vivaldi. I now have a functional save dialog.
Please give it a try and confirm.
Best,
Bill