Start dev with purely docker, or something like vagrant (in a to-be-dockerized application)?
Vagrant, in this case, would simply add a lot of unnecessary overhead into your testing environment. You can create containers that quickly test specific groups of functionalities and paste the results somewhere. Your team won't spend much time learning how to start these containers as it is usually a simply command. Most of changes are going to be done inside the code, not into the testing environment. But if you need to change the test environment, the Dockerfiles must be in an easy to access place. You could also automate the build process of containers linking the Dockerfile (e.g. from a GitHub repository) into the DockerHub or Store.
I can't see a reason here for not going into Docker. This knowledge will be more usefull for your team than Vagrant skills.

You can read more about it here: https://stackoverflow.com/questions/16647069/should-i-use-vagrant-or-docker-for-creating-an-isolated-environment