Welcome!
I don't have a VX, but like mpg numbers guy I have a manual Insight with lean burn. We're blessed with an OEM mpg gauge. Is the MPGuino even available?
MPGuino trip computer - MPGuino suspended orders a year ago.
I seem to recall something about a federal ecu vs a California one allowing more lean burn.
Again, a wideband o2 gauge would show when you're in lean burn without the fun of seeing big instant mpg numbers. Might be your best bet. I don't think you need to worry so much about how accurate it is, you just need to monitor the jump in the air/fuel ratio.
In my car it seems to need to be up to temp, outside temp isn't low, no loads like ac/defrost, throttle under 19% (shown by the torque app/obd2 wireless adapter) and under 2500rpm.