The root cause is that VirtualBox does not cascade the file change events on the host to the VM. Guard (on Linux) is expecting to receive notifications via inotify events. Instead, you can have guard poll for events with guard -p, but this can lead to your CPU maxing out, so you can throttle the polling back down with guard -p -l 10.
From guard help start:
-l, [--latency=Overwrite Listen's default latency]
-p, [--force-polling=Force usage of the Listen polling listener]
http://www.softr.li/blog/2012/07/21/running-guard-over-vagrant