Vagrant stores the state of the machine and machine id inside the .vagrant folder. The running machine (vm) itself is handled by virtualbox/vmware or any provider your using. Lets say the virtualbox box is stored somewhere else on your system and referenced by Vagrant.
If you access the folder from two systems your basically remote controlling two different machine on two different systems. Not a good solution. Furthermore, you will run into problems if the states are different, e.g. its "up" on system one but "destroyed" on system two.
Additionally to the above solutions I propose the following:
- Vagrant Share! Enable Vagrant http-/ssh-Share between your systems.
- Vagrant machines should be repeatable and destroyable. Therefore, put your Vagrantfile under version control and checkout on the two systems.
- Configure your provider to store the box itself on the dropbox.