From my experience there are a few ways to get the most fe from a vx. The first thing that takes some getting used to is the engine that can run at very low rpms. It will actually accelerate smoothly from 1k rpm if you are very light on the gas. If you have the shift indicator light it will tell you to shift into 3rd at 19mph, 4th at 25, and 5th at 29. If you need to seriously accelerate this will not happen at 1100rpm, but under many conditions, even in traffic, you can pull of gentle acceleration from 1,100 or 1,200 rpm. Once you have the lean burn, you can try to keep it in lean burn by letting off the gas a bit and losing some speed on hills. also, since it is not super aerodynamic, going above 55 drastically reduces your economy. I would guess the mpg would be impressive if you could drive a tank at 33mph in 5th, but i am not patient enough. The low rolling resistance tires should help quite a bit on a car so small and sensative as the vx. I cant wait for mine to wear out! enjoy!
I run synthetic amsoil syncromesh manual tranny fluid in mine, 8k since i put it in with no problems, but cant vouch for its long term compatability. Also have mobil 1 synthetic in the engine. I would say maybe 1-2 mpg for both, but with the vx fuel tank it is really hard to tell.