I read your comment in more detail, you’re going down the wrong path. What you’re looking for cannot function the way you want the way you want to achieve it, and may not even make sense to want. I am wrong, I didn’t realize Caddy could just serve their cert over the socket. What user is the caddy process on your VM being run as?
If you want to use Tailscale DNS, you can use their TLS cert (assuming it gives a valid cert for machine.domain.ts.net) and just reverse proxy HTTP traffic with nginx on the VPS/VM (assuming nginx can listen on their network device. I’ve fought with that with openresty before, but that may be because I was trying to host it in another docker container lol).