Some parts of this make sense, others are more obscure. For now, what's included here works. My VirtualBox host is an Ubuntu 14.04 server. I used apt-get-install for VirtualBox and Ansible, and installed Vagrant from a Debian package so I could get a fresher version than the official Ubuntu version.
I'm a bit mystified by hosts in Ansible. For now my solution is to use "all" since I'm currently only setting up one machine, but I'll need to be smarter before too long.
So I'm working mostly from this blog post to clear up my confusion around hosts. I've already spun up three machines but they're not networked together yet.