One difference is that Vagrant is a full stand-alone server VM rather than a shared container like Docker, so Vagrant is better for housing multiple services in a single Vagrant instance, while Docker is better off housing a single-use application/service.

Example: If you wanted to run a complex local dev environment with apache, mysql, memcache, varnish, etc... You could run that all in a single large VM (provisioned and booted by Vagrant) or by multiple Docker containers (one for each service).

I don't know about OS X but on Linux Docker is so much better than Vagrant (not really, it's the VMs). But that's just my opinion.