Who is responsibile for copying the cookbooks to the specific Vagrant folder (Berkshelf, the vagrant-berkshelf plugin, or Vagrant)?
A little from column A and a little from column B, both of which get some help from column C.
The Vagrant chef-solo provisioner has the ability to copy cookbooks (+ roles/data bags/etc) from the host machine to the guest. It does this by creating a directory on the host and then mounting that directory within the VM.
The vagrant-berkshelf plugin first creates a directory on the host and then uses vanilla Berkshelf functionality to export all of the relevant cookbooks to this directory.
Lastly the vagrant-berkshelf plugin tells the Chef-Solo provisioner to use the directory it has created as the source for the cookbooks it should copy across to the guest.