Its doubtful the phone is not vibrating due to a hardware problem or "bad" phone. It's much more likely that its a software setting or issue. If it's a new phone, you have 90 days of free phone support through apple. Call them at 800-MYIPHONE and an Apple Engineer will quickly walk you through checking all the possible causes to rule out a software issue. If it turns out to be a hardware issue, they can arrange for a replacement device.