Note the variety of preset vibration patterns here! Tap "Create New Vibration" to customize your own.
As you tap in the field, the phone will vibrate and give you a visual "splash" indicator. The progress bar shows the vibration pattern as you go.
Tap "Save" in the upper right, give it a name, then "Save" again in the dialogue box.
Now assign that custom vibration to whatever contact you'd like!