Here's some details for anyone else who may want to try sharing a flatpak vivaldi between linux distros running different "secret services".
To see what "secret service" your distro is running, you can run, if the command is
available or installable:
/usr/lib/qt6/bin/qdbus --session org.freedesktop.DBus / org.freedesktop.DBus.GetConnectionUnixProcessID org.freedesktop.secrets
Failing that, you can try this: open a command line window and try:
ps aux | grep gnome-keyring-daemon
OR
ps aux | grep kwallet
to see which one is running (if any).
(Note you'll always get a hit on the grep itself, but ignore that one.)
While in a system running kwallet:
start kWalletManager from the menu
under "Folders" click the "+" next to Chrome Keys
click the "+" next to "Passwords"
click "Chrome Safe Storage"
then in the right hand panel, click "Show Contents" and the key is visible (and changeable, so be careful)
While in a system running gnome-keyring-daemon:
start "Passwords and Keys" (also called "Seahorse") from the menu
go to Passwords - Login - Chrome Safe Storage
view the key by clicking on the key icon (or copy it with the "copy" button); this
one is changeable too, so use due care
So you want to copy/paste the key from one of those into the other, depending on which system has the already working Vivaldi. (Be sure Vivaldi is NOT already launched on the system you are modifying.) Once you've pasted, you can start Vivaldi.
The fortunate part here is that Vivaldi (Chromium core) is ONLY using the distro's own "secret service" for the storage of its key; all the encryption is done internally to Chromium core, based on that key. So just use the same key and everything works.