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

There is actually a much simpler solution. See https://gist.github.com/colindean/5213685/#comment-882885:

"please note that unless you specifically want scp for some reason, the easiest way to transfer files from the host to the VM is to just put them in the same directory as the Vagrantfile - that directory is automatically mounted under /vagrant in the VM so you can copy or use them directly from the VM."

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>