Your Vibe is a Toyota product so you may run into the same thing I did;
With key on, engine off, car moving,
the
SGII reads .05 gph. (? sometimes a little more???)
I don't know why, but other Toyota owners have the same issue.
It won't give you 99999 but will give you an inflated mileage value.
I get readings over 1000 mpg when gliding on the highway.
ie: 60 mph x .05 gph = 1200 mpg.
Your trip mileage will go up
Your tank mileage will go up
just not quite as much as if you were showing 0 fuel at all.
Hope this helps clear some stuff up.
S.