While digging into why Vagrant didn’t setup a “more secure, but insecure” private SSH key pair for one my boxes, I “lost” the public key file and had to extract it from the private key part. This is how I did that:
ssh-keygen -e -f id_rsa -m rfc4716
(Vagrant somehow made .authorized_keys end up with garbage carracters in the beginning of the file, which I’ve not seen before.)