vagrant: relocate vbox vms

documentation/vagrant.markdown

@@ -16,6 +16,13 @@ Note: on our managed Linux workstation Vagrant is already installed.
 Preparation
 -----------
 
+When using VirtualBox you need to re-direct the path where virtual machines are stored outside of your home to the local disk.
+
+```sh
+VBoxManage setproperty machinefolder /scratch/some_path
+VBoxManage list systemproperties | grep 'Default machine folder'
+```
+
 Start by adding an image of a preconfigured virtual machine to your system. You can find our collection of compact Vagrant boxes at https://share.phys.ethz.ch/~isg/vagrant/.
 
 ```sh