My iPhone4 has the softest vibration in silent mode of any phone I've ever used...and much less than the 3GS I sold to get it. I am missing calls left & right simply because I can't tell when my phone rings.
Is there an app...free or not...that will dramatically increase the vibration?
There is no, and never will be an app in the AppStore that will change a setting like this.
Wait until the phone can be modified, then maybe you will be in luck. You will be able to adjust the length of vibration, and MAYBE the speed and effectiveness.
Right now you have to stay put though. Also any case will further damper the vibration.