If you push the button above the volume control it will silence the phone and yes - it should be on vibrate then. It's not the strongest vibrate though. Try it out. You should feel it vibrate when you push that button.
i don't have an iPhone but ive seen it in a video. under settings there is a slider option for vibrate on silent and for vibrate on ring. im guessing if vibrate is on for silent, when you push the button on the side for silent it should still vibrate. try that.