UIViewport is a helper script that can set the camera's viewport rect. I am not sure what it is you're trying to do with it, but if you want one camera to match another, they both have to have identical viewports. Furthermore, setting the viewport doesn't change the camera. If your screen is 1280x720, but viewport rect is 1200x720, then you will have 80 pixels of padding somewhere on the sides.
Viewport just alters the drawing area. Technically this isn't an NGUI question at all -- I recommend checking Unity's documentation for the camera class.
