It depends on your perspective. Vagrant is billed as a manager of development environments. It has a well known and easy to use workflow for creating/modifying/destroying those environments (vagrant up/provision/destroy). With its Docker integration, it provides you with a nice management layer for managing those Docker development environments. You could achieve the same thing with Docker directly, but there would be some non-zero amount of work required to create your workflow commands and share that development environment with others (whereas with Vagrant you just need to share a Vagrantfile).

Here's an interesting Stack Overflow question related to this topic, http://stackoverflow.com/questions/16647069/should-i-use-vag.... Also, it looks like Docker is working to build out its own development environment tools using Fig, but that seems a bit nascent right now.