I have an 09 AWD vibe. The gas mileage is good in the summer. I get roughly 480-500 kms a tank if mostly highway driving. The best way to get more MPG's with these is to drive at 100 KM/Hr. It does seem slower but I got as low as 8.3l/100 kms. Driving at 120 gets you 11.7l/100kms. Wintertime, because of the winter fuel and winter tires, it gets 280-300 a tank. That's with a 27 km commute to work in the morning, mostly secondary roads and highway driving, no stop and go traffic. I don't like the winter mileage really..
I love the AWD in the winter. It's very stable in snow and ice and has excellent handling characteristics because of the AWD's rear suspension, which is essentially a Toyota Celica suspension setup I believe.
My brother has a FWD 09 Vibe and it has the 2.4 and a 5 speed. He is reporting 450 a tank in winter. He has a real nice setup, but doesn't have as slick of a rear suspension as I do, but he does get decent winter and summer mileage over me.
That 2.4 is a really good engine, but it does use oil, and I've noticed it is no different than the 2.4's found in the Camry's from 02 to present. It is a really nice silky smooth refined powertrain. It also hauls quite a load if you need it to. I guess that is the trade-off.