If you have run the command below - it will not start nor enable by itself.
sudo systemctl disable --now systemd-networkd.service systemd-networkd.socket
If it does enable - then you have other issues - perhaps the vpn requring the systemd-networkd service - suffice to say - nothing enables by itself - it must be triggered by something.
You will have to backtrace - e.g. check if your resolv.conf get’s restored correct after disconnection your VPN - yes I saw you didn’t reckon the VPN was part of the problem - do check the resolver status and check if the lack of internet is due to missing DNS or the NIC being down…