Your mileage doesn't seem very far off from what others on gassavers are getting if you look at the other VXs in the garage. A new oxygen sensor may help a little, but as far as it being "hard to get less than 60", that is unrealistic. If this was the case there would be many more people with 60+ MPG 90 day averages in the garage. Looks like many people are in the low to mid 50s for an average. It would take awhile to pay for the O2 sensor if saving money is your main object.
If you haven't seen it already, this site seems to have a good deal (only $195) on an O2 sensor for the VX:
http://www.oxygensensor.net/l1h1.html