Virtual LANs give you the ability to sub-divide a LAN. Linux can accept VLAN tagged traffic and presents each VID as a different network interface (eth0.100 for VID 100)
Previously Arch Linux used vconfig to setup VLANs. This had been superseded by the ip command.
Make sure you have iproute2 installed
# pacman -Ss iproute2
Adding a VLAN
To add a VLAN use a command with this syntax:
ip link add link INTERFACE name INTERFACE.VID type vlan id VID
ip link add link eth0 name eth0.100 type vlan id 100
Run ifconfig -a to confirm that it has been created. This interface behaves like a normal interface. All traffic routed to it will go through the master interface (in this example, eth0) but with a VLAN tag. Only VLAN aware devices can accept them if configured correctly else the traffic is dropped.