After updating your VirtualBox installation might get the following message after when starting you box with
'The guest additions on this VM do not match the install version of VirtualBox!'
Most of the time everything will keep working just fine and you can ignore this message. But when it doesn't (ie. your mounts won't work anymore), you need to update you guest additions.
Fortunately there is the Vbguest plugin!
Just go to the directory where your Vagrantfile is located. And type the following:
$ vagrant plugin install vagrant-vbguest
Now every time you start you box with
vagrant up your box will check if your Guest Additions version matches the one of your host, if not, it will update them automatically.
More info on the Vbguest plugin can be found on their Github repository: https://github.com/dotless-de/vagrant-vbguest