I'm just going to go into detail about the Vibrant vs Nexus S decision. To start off, the Nexus S is by far the better choice. You'll find that the Nexus S has very similar hardware to the Vibrant. In fact, the share the same processor, screen, etc.
The Nexus S has a few benefits though, such as the front facing camera, and a working GPS sensor. The Nexus S is also a pure Google phone, meaning it'll be among the first Android phones to get any updates (compared to the Vibrant which is still on Android 2.1). As Google's new development platform, the Nexus S should also run Android better than other devices with similar hardware. To top it off, the Nexus S has a better camera and LED flash.
Now the Vibrant has a couple advantages. For instance, the Nexus S has no expandable storage, so you're stuck with the 16GB the phone comes with. The Vibrant can also record video in 720p, while the Nexus S is stuck with 480p, at least for the time being.
Basically it comes down to this:
Nexus S
+Working GPS
+Better Camera w/ flash
+Front Facing Camera
+Runs Android better and gets updates first
-No SD card slot
-No HD video recording (at least for now)
Vibrant
+SD storage (on top of 16GB of internal storage)
+720p Video recording
-Poor GPS
-Poor camera for low light conditions
-Very slow to Android updates. TouchWiz slows things down.
Also, I feel that the Nexus S feels a lot better in the hand. It has a bit more weight to it, but it's a good weight and makes the phone feel more sturdy.