Check to see if your brakes are hanging as well. that would be the number one cause for the low FE after the other aspects have been eliminated.
The fastest way to check the rears is to jack the car up by the tow hook in the back. Be sure to use a nice hydraulic jack with a wide lift point on it for safety. Once the back tires are in the air give them a spin. With the E-brake not on and good wheel bearings the wheel should spin for at least five revolutions with a good spin.
For the fronts, I always take the car for a drive of about 15 minutes. Then check the temp of the rims in the center to see if there is a difference or extremely high temp. You can also just jack up the front and remove the tires and check by site on the rotors for scoring and darkened/blue marks and minuscule cracks (from too much heat).
you might check that your timing belt was put on correctly, normaly when your valve timing is set, you aline the notches on the cam pully, with the top of the head, but the VX has two little arrow pointers that put the cam pully almost a whole notch off, the car will run and drive, but not have a whole lot of power, and get lousey mileage, to check this it's just a matter of taking off the valve cover, and the plastic timing cover, and alining the marks on the cam pully, and the crank pully.
i followed TomO's method to check the o2, if anything it may be reading a little more lean then his numbers suggested, is that reason enough to replace the o2? ive been watching ebay for a good deal on one
the sparkplugs that came out were not that old, however on the insulator, 75% was the original whitish color, but there was a black spot on each insulator about the size of a pea. now the plugs that came out were the wrong type for the vx, the vx required the zfr4f-11, the ones that came out were the zfr5f-11 according to the previous owners receipts.
i will have to check the brakes and see if i can check/adjust the timing.