My wife's last vehicle, a Mitsubishi Outlander, came with a 2.4L 140hp banger. It averaged 17mpg.
That little, underpowered wheezer (same as comes in the Galant, Eclipse, etc.) was just plain working too hard trying to push the 'ute around.
I suspect it is the same with the VX.

Spazz,
I've admired the pics in your gallery for some time (excellent references to the location of various bits of the VX -you have a talent for taking pictures of the right things), and was under the impression that this mileage drop was a result of all of the mods.

Granted, the VX isn't the greatest on gas, but then again I would've bought a hybrid if that were my priority.
I think that it's mileage is respectable when you compare it to the other vehicles in it's class -think Jeep xj/yj/tj/zj/wj or a Series II Discovery.