If a vagrant virtual box blocks system shutdown ...
I forgot to close my dev vagrant trusty box a hundred times already, everytime keeping my machine in endless waiting state when shutting down, forcing me to hard reset the damn thing.
To circumvent this, just edit /etc/default/virtualbox and add
SHUTDOWN_USERS="all"
# i use acpibutton here instead, which shuts the virtual box down normally
SHUTDOWN=savestate
to the end.