Is this using the normal VirtualBox "shared folders" functionality? For Vagrant we had to drop VirtualBox in favour of VMware Fusion because VirtualBox suffered cache corruption almost every day. You would write a file on the host, and the file would be corrupt inside the VM. Last I checked, this bug was still open, but I'm not certain (on my phone right now), but it still makes me wary of using VirtualBox again. Have you dealt with this issue at all?

Edit: Or is this not using any VirtualBox code at all?