Today I learned that for self-signed certs you need to add the root CA of your server to Vagrant’s cacert.pem
On my system this file is at /opt/vagrant/embedded/cacert.pem
The certificate for Proxmox’s self-signer root is at /etc/pve/pve-root-ca.pem
Just copy the contents of pve-root-ca.pem and paste it at the bottom of cacert.pem. This should resolve issues with Vagrant and self-signed certs.