While using Vagrant (I'll leave the question "Why should I use it?" for some future post) and taking ready made Boxes from http://vagrantbox.es - the fist issue I have faced was a warning about inconsistency of VirtualBox Guest Additions and actual VirtualBox version I am using.
Why is this happening ?
Because VirtualBox Guest Additions - it is drivers etc. installed inside the Box (that you are downloading somewhere) and for each new VirtualBox version new Guest Additions are required.
It can work with old version in general, but very essential parts could fail - like shared folders etc.
How to fix it manually
You could go to VirtualBox downloads server and find yourself needed ISO, download it inside Vagrant box via wget for example, mount it and install. But it is boring :)
Autoupdate Guest Additions via Vagrant plugin
vagrant-vbguest comes to help us with it.
vagrant plugin install vagrant-vbguest
Add one line in your Vagrantfile
Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
config.vbguest.auto_update = true
and restart Vagrant box
If you are lucky - everything else will happens automatically and you will be notified about version of Guest Additions that is being installed.
Also go through Configuration docs for vbguest to see other useful params.