modprobe vboxguest failed

I had this problem today, and solved it by installing the kernel-headers

yum install dkms binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers 

then

yum install kernel-devel

or

yum install kernel-PAE-devel

Then re-run VBoxLinuxAdditions.run.

I discovered here that the standard guest additions install includes a built-in setup function, so there's no need to reinstall:

/etc/init.d/vboxadd setup

This gave me the error about missing kernel sources, so as brendan's answer, yum install kernel-devel, and then that setup command works, and everything seems to be back to normal again!

I had a similar problem today, but it turns out the vagrant-vbguest plugin was not installed. So, in the folder with my Vagrantfile:

vagrant plugin install vagrant-vbguest

That solved it for me.