That's the good thing about containers, that's super easy. Just stop the container, move your folder to your desired location (e.g. /home/user/vaultwarden), change the path in your compose file (stack) (i.e.: /home/user/vaultwarden:/data/), redeploy the stack and that's it :)
Just to correct a mistake: - ./vaultwarden:/data/ means that the folder /data/ of the container is in the subfolder vaultwarden inside the folder that contains the docker-compose.yml. it is not located in /. for that you need to remove the leading "."
If you remove ./ vaultwarden points to a volume named vaultwarden that need to be defined separately:
./vaultwarden = relativ path from the docker-compose folder
This is good. there was an OLD vaultwarden-folder in my root-directory and i thought this would be the current, but you are absolutely right: The folder is in the compose-directory!
It's more like quick and dirty. I generally try to create a volume and save the data outside the compose folder. Default is /var/lib/docker/volumes if I remember correctly.