Hello Weisong.
Can you share more details?
Vagrant will use Vagrantfile and the other files in your project, for those you usually use a control version software like Git.
If you are talking about VM, then I will suggest check the documentation of the virtualization tool. ie Virtualbox.
Doesn't make much sense backup a VM, since Vagrant should be able to provision a new one again, and the projects of the application should be separate from the VM itself, so at any point you should be OK with delete and recreate the box.
Can you share more about the use case where you need to keep that VM?
Thanks
Alvaro.