Not the one who wrote initially, but i have the same setup (mostly).
I went with a self signed certificate. So the server is running with a certificate i have signed with my own certification authority certificate (ca-cert) .
That means I have to install the ca-cert on all devices to get vaultwarden to accept it.
The alternative is a let's encrypt cerrtificate, which are free, but you need to open port 80 (and another one if I remember correctly) for it to work (at least every 3 months)