[vagrant] Should I use Vagrant or Docker for creating an isolated environment?

Definitely Docker for the win!

As you may know Vagrant is for virtual machine management whereas Docker is for software containers management. If you are not aware of the difference, here is: A software container can share the same machine and kernel with other software containers. Using containers you save money because you don't waste resources on multiple operating systems (kernels), you can pack more software per server keeping a good degree of isolation.

Of course is a new discipline to care with its own pitfals and challenges.

Go for Docker Swarm if your requirements cross the single machine resources limit.