You won’t find anything there either. To simplify installation, we have created our own DietPi Debian package, which we use to install Vaultwarden. The certificates are also created during installation of the Debian package using apt.
You could now go ahead and delete the current certificates manually and then reinstall Vaultwarden via apt. The configuration and user settings, etc. should remain intact. Only the certificates should be recreated.
rm /mnt/dietpi_userdata/vaultwarden/*.pem
apt install --reinstall vaultwarden