Isolation in Vagrant vs. Docker

Isolation in the meaning "only isolation" i.e., not virtualization. When you want to run linux apps on linux, we are talking about isolation; when you want to run any app on top of any os, then we talk about virtualization.

Where did you read that Vagrant was not considered an isolation?

Actually, this statement is true as Vagrant is not a Container backend nor a VirtualMachine. It is a manager. It can manage VirtualBox, VMWare and now Docker. Depending on what your needs are, you can achieve isolation through Vagrant via VirtualBox or Docker, but Vagrant does not provide the isolation by itself.

Now that Vagrant supports Docker, you can use it if you need to; however, Docker is very simple by itself and IMHO does not require a tools like Vagrant. When you are playing with Virtual machines, on the other hand, Vagrant is very helpful.

Vagrant is just a tool to create virtual machines (or even cloud instances and Docker containers). Vagrant itself does nothing for isolation. However, the tools it can handle (such as virtual machines or Docker) can be used for isolation (but also for many other things, isolation is just one of many aspects).

For some enlighment about the difference between Docker and VMs see How is Docker different from a normal virtual machine?

Docker: separates the application from the underlying Operating System that it runs on.

Docker virtualise the Operating System for the application.

Vagrant is a virtual machine manager, so let's compare the Virtual Machines to Docker.

Virtual Machines: separates the Operating System from the underlying hardware that it runs on.

virtual machine virtualise the hardware for the operating system.