So all this meant a lot of fiddling. But the cool thing about FOSS is that the maintainer of the Vaultwarden Yunohost integration added the Dex auto-config in a test branch this morning so it worked great so hopefully it will be available in main soon. Oh and last but not least, if you setup Vaultwarden on a sub-domain, you need to enable outgoing email on that sub-domain as it needs to send email notifications. Conclusion: Vaultwarden is not quite plug and play yet. 3/3