That is low, even for city driving in Houston, with our horrible 'EPA' mandated low mileage gasoline.
My first thought would be the MAF (Mass Air Flow Sensor)
Second would be the wrong spark plugs. It turns out that the Vibes are VERY picky, and if you put in the wrong ones, they run, but not as well. I found that out in 2004-5
That's about all I can suggest, as I have no experience with the post 2008 Vibe engines.
I've also noticed you posted this in three forums, so it might be worth consolidating them. You also haven't said where you might be; there might be someone reasonably close by that can assist.