They're not mutually exclusive. Vagrant is for providing a copy of the production environment on a development machine easily and reproducibly. If you use Docker in production, then you should also include it in your Vagrant image. If you don't, then don't. If your goal is just to provide a copy of prod for developers, and you don't currently use Docker, then you probably just want Vagrant. Good presentation (sales pitch? :D) by the creator Vagrant: https://youtu.be/Im4wNqlolqQ

Security and stability are definitely big reasons. Additionally, Linux has a more administration tools and technologies. Docker for example pretty much only works on Linux because of Linux Containers. You also have more choices for things like file systems. Oh and being free definitely helps.

.io is a geographical domain for Indian Ocean.

"In JavaScript, you could write a Web server in [five] lines of code. No way you could ever do that in Java," Belder said at the conference. That's extremely misleading.