Do you realize that you’ll need a MIDI interface for your iphone right?
They suggest the i-MX1, you have on the same line the iRig MIDI and the line6.
I was about to suggest you to use TouchOSC on iPhone, but I think that you can access the vibrate function just by using OSC, not by using plain MIDI.
Now returning to your original idea of using a sample to trigger the vibrator.
I think that you can use a positive only pulse sample. Then make a kind of amplifier to convert that pulse to a range/kind signal able to drive the vibrator. I think it could work, and I guess it will be cheap.
Then you can use different pulse lengths and you’ll get differentiated vibrations, so you can recognize the bar start, etc. (like morse code)