Easiest way to copy a single file from host to Vagrant guest?

Since you ask for the easiest way, I suggest using vagrant-scp. It adds a scp command to vagrant, so you can copy files to your VM like you would normally do with scp.

Install via:

vagrant plugin install vagrant-scp

Use it like so:

vagrant scp <some_local_file_or_dir> [vm_name]:<somewhere_on_the_vm>