Firstly install Virtualbox and Vagrant:
sudo pacman -S virtualbox vagrant
If you met the following warning, and vagrant can't start:
The vboxdrv kernel module is not loaded. Either there is no module available for the current kernel (3.16.4-1-ARCH) or it failed to load.
run following commands to compile kernel module vboxdrv and load it:
sudo pacman -S linux-headers virtualbox-host-dkms
sudo dkms install vboxhost/4.3.18
sudo modprobe vboxdrv
Verify the module is loaded successfully with lsmod |grep vboxdrv.
Ref: VirtualBox on ArchWiki.