What's the middle terminal in the VN2222 MOSFET?
The middle terminal is the substrate (edit: also body or bulk as Null says). 

In almost all discrete MOSFETs (including the VN2222) it is effectively connected internally to the source. It behaves a bit like a second gate (though not an insulated one) if you had access to it. You should connect it to the source if you're trying to simulate a real VN2222. 

An important application that requires the substrate not be connected to the source is the analog switch: