Warning:
[default] The guest additions on this VM do not match the installed version of
VirtualBox! In most cases this is fine, but in rare cases it can
cause things such as shared folders to not work properly. If you see
shared folder errors, please update the guest additions within the
virtual machine and reload your VM.
Guest Additions Version: 4.2.12
VirtualBox Version: 4.3
Fix
Update your virtualbox guest additions. But sometime after upgrade, vagrant still shown us warning like we seen before upgrading. Run this command to prevent warning shown up again.
VBoxManage guestproperty set <vm_id> /VirtualBox/GuestInfo/OS/Product Linux