The custom vibration implementation so far is pretty nifty, but it needs a more fine grained, detailed control. It would be easier to get the exact pattern a user is looking for if they started by entering it the way it is now, then transitioned to a textview to set the timings exactly how they want them.

Alternately, if you want to do a bit more work, some form of slider with tabs at the beginning and end of the vibrating segments that the user can move would probably be a bit more user friendly, but personally I'm cool with just plain old comma separated on/off values.