What is wrong with my Vibe - only getting 20mpg in city (1.8 awd)

Depends on how you drive, what kind of oil you're running, how often you change the oil, what kind of filters you use and how often they are changed, hundreds of factors. Is it behaving normally?

Yes, although when I turn it up, it idles at about 2,000 rpm for a minute or two, I guess that might be a small factor. .. I will be changing the oil soon, and will put in synthetic ... but that wouldn't contribute more than a small percentage, no? .. I have a new air filter, and I cleaned the MAF sensor, but have not yet touched the plugs ... this car should be getting 50% more mileage ...

Synthetic should improve it some. Definitely check the plugs, look for any kind of fouling or deposits and find something that tells you how to "read" a plug, that could point out a problem that isn't immediately apparent.

DONT go to synt oil it will start useing oil,change your plugs and wires clean the IAC,idle air control,that is why it is going up to 2k on idle,is on TB,1 plugin round clean with carb cleaner,spray a little wd 40 in it replace,they get carbon build up in them and cause them not to work right,when removed DO NOT push in on the round part sticking out of it.

It is not going to get way more than that- they are rated for 26 city and 31 hwy under ideal condtions- if there is a lot of stop/starts, that will cut it back- you may want to run some fuel injector cleaner through the gas tank too to make sure things aren't getting gummed up.