He was talking about Vagrant (the tool CoreOS uses to distribute VM images)...
Also, I think you can just make a directory in the `~/vagrant.d/boxes/coreos` folder called "vmware" and copy over the vagrant file, box.ovf and the metadata.json. Then you just edit the metadata to have a vmware provider and the box.ovf to reference "../virtualbox/<<theimage>>"
I however do not have a vagrant vmware license and cannot test this theory.