There are still times I look to Vagrant instead of, and alongside, Docker.

The reason being Docker for Mac uses a VM anyway (an xhvye machine) - it does try to hide/abstract this away, but inevitably this leaks. The xhyve VM has the usual parameters memory, diskspace, CPUs, and not least a kernel. There are limited options to fiddle with these parameters, though you can log into it and poke around there. I thus find it easier to just have setup Vagrant machines with Docker - then I have better control over those things.

If I were on a Linux distro though, I'd probably use Vagrant a lot less.