I have Vagrant (1.7.4) configured with the default configuration from vagrant init using the latest version of VirtualBox (5.0.16 r105871) on OS X (10.11.4) and the
vagrant init
This is more likely to happen if the VM was shutdown or rebooted from within the VM.
Halt the VM using "vagrant halt" and start the VM again using "vagrant up". This is should automatically mount it under /vagrant