Please note my signature before continuing on...
I would keep the Cruze. I've actually been looking for a Cruze Eco and just barely missed out on one at a screaming deal (~3k under market value). I think your car, based on my area, would get sub-10k on trade in at a dealer. Local prices are around 12k-14k at dealers. The stick is definitely an issue, around here.
What you have is still a rarity at this point in time. High mpg, but a decently built, solid, substantial, modern, safe car. Only newer choices in the 40 MPG area are hybrids, Fiesta 3 cylinders, Cruze Eco, Hyundai Elantras, and VW diesels (there are a handful of others but they are too oddball or old - e.g. early 2000's Honda Insight - for me to group them in). The Fiesta is just fine for one person but gets a little crowded with more, and the safety ratings are not near what the Cruze has attained. Hybrid versions of family sedans that can get close to 40 MPG are much more expensive on the used market.
Nothing against the Vibe but I wouldn't expect more than 33 mpg while a properly driven Eco can get over 40, and there is a tune that is supposed at add up to 3 mpg on top of that. Vibes are hard to find around here that aren't all rusted to hell but that's probably not an issue for you.
Just my $.02, if you are financially sound, I'd stick with the Cruze.