See BlackDex’ explanation: I cannot figure out how to source the .env file - #4 by BlackDex
The .env file mentioned in the logs is unrelated to the environment file used by docker (or systemd). While launching, vaultwarden checks for the existence of such a file in the current working directory of the container and simply informs you if it does not find it.
You could probably bind mount your vaultwarden.env file to /.env instead of specifying it as environment file for docker if you want to get rid of the message.