Basically you're going to need to tell your router/switch that the Arduino's Ethernet port is a trunk port (or does trunk autosensing), then you're going to need to manually write your Ethernet frames to have an 802.1Q header before the Ethernet Type field:http://en.wikipedia.org/wiki/IEEE_802.1Q
and when you receive packets you'll need to manually decode the frames to pull out the vlan tags.
I don't think you'd be able to use the regular Ethernet library to do any of this; you'd have to write your own interface to the WS5100. I looked through the WS5100.cpp and .h files and didn't see anything about raw frames so I'm not sure it's even possible, but I didn't dig that deep so may have easily missed something.
In any event given the huge RAM etc. limitations on an Arduino I can't think of why this would be a good idea to try other than "because I can", but please let us know if you pull it off -- it would be very impressive.