Given that many points of interest in the city are well-illuminated at night, relying on Vibration Reduction (VR) and pushing up the ISO slightly can be a good fallback. Even though there is no shortage of poles, railings, and scaffoldings for mounting a Gorillapod, they aren't always in the most convenient spot for taking a photo. (On that note, I wouldn't rule out a monopod, though a tripod would be ideal.) I was curious about just how much ambient lighting could minimize shutter speed, so I took some snapshots yesterday without a tri/mono/etcpod and using a non-VR/IS lens, while trying to keep ISO as low as possible (100% crop inserts).
f/4.0, 1/10 s, ISO 100, stabilization: pole
f/4.0, 1/3 s, ISO 200, stabilization: railing
f/4.0, 1/15 s, ISO 200, stabilization: chair
DPReview's tests of the 18-55mm kit lens show that its VR is capable of managing shutter speeds down to 1/3 s - provided good shooting technique is used - allowing for reasonably low ISOs in lighting conditions like the above. Pushing the ISO higher to 400-1600 would make handheld shooting even less of a worry.
Since nobody mentioned it yet, why not just go without one and see if you need it? NYC doesn't have a shortage of camera stores, and if you find you do need a flash, walk down to B&H (which is an experience in itself) and buy yourself one. Plus it's probably cheaper there than if you were to buy it elsewhere and bring it.
B&H and Adorama (and other stores that observe Passover) will be closed from April 7-13 (on the 6th, B&H will be closed and Adorama will have shortened hours), which may be an issue for the OP, depending on what "the week of Easter" (which on the 8th) means. Otherwise, good suggestion.