It depends on which version of the Galaxy S Vibrant you are talking about. If you are talking about the Bell version, then yes - there is a front facing camera. The app will select the proper camera when needed.
For example - when you fire up "CAMERA" to take a picture, it will default to the rear camera so that you can take a photo like normal. In the camera app, you can switch it to take a "profile" or "self picture" - then it will flip to the front facing camera and you will see yourself and you can snap a "proper" pic of yourself for other things.