When was the last time the thermostat was replaced? My VX is super sensitive to temps above 180df. I ran a 195df t stat for a while and it would barely keep lean burn active. That'd account for some significant mpg loss.
Ok, update time. The one tank at 53 mpg was apparently a fluke (almost all highway, and I kept it at 55mph most of the time). I kinda dropped the ball on pursuing the problem for a little while, but I decided to sell the car after finding a killer deal on a Honda Insight that I couldn't pass up. So I went to take the VX in for smog and it failed miserably. High HC and NOX. The owner of the shop is a friend of mine and after talking for a while, we've come to the conclusion that I need a new catalytic converter regardless. However I decided to really look into why the car has been getting 10-20% less mpg than it should, because obviously something must have contributed to the cat failure and I'm betting it's related. The only real problem I found so far was the vacuum hose coming from the intake to the pcv valve in between the runners had a subtle leak that actually sucked the hose in on itself, blocking flow. After fixing that, I've hooked up a vacuum gauge and everything checked out ok with that aspect. I've never replaced the thermostat, so I can't say for sure if that's correct, but I know I can feel the car going into and out of lean burn mode depending on throttle position on the freeway. The last thing I can think of doing is having someone hook up a Snapon scanner to monitor what the ECU is doing while driving around. Anybody else got some ideas? I've tried messaging thewizard182 because the symptoms he describes with his VX seem remarkably similar, but haven't gotten a response.