OK, so as in my comment above, I suspected Ubuntu's power management for shutting down the machine rather than Vagrant. As you probably already know, you can set the action for when battery is critically low to 'nothing' (as found in another answer):
gconftool-2 --set --type string /apps/gnome-power-manager/actions/critical_battery nothing
Also, here I have found the same hint.
Additionally, you can play with other options if you like (found here):
/apps/gnome-power-manager/general/use_time_for_policy (false)
/apps/gnome-power-manager/thresholds/percentage_low (12)
/apps/gnome-power-manager/thresholds/percentage_critical (8)
/apps/gnome-power-manager/thresholds/percentage_action (5)
/apps/gnome-power-manager/actions/critical_battery (hibernate)