Open Phone or Contacts; select a contact, then tap the Edit button in the top-right corner. Scroll down to find the ringtone field; below it is a vibration field. Tap that, and you’ll see an assortment of built-in vibration patterns you can choose from. Further down, though, is the ability to add a custom pattern: tap Create New Vibration, and you can tap on the screen to create your own rhythm. When you’re satisfied (tap the Play button to see what it will feel like), tap Save to set the pattern.
If that's not enough, go back to the contact and also assign a custom vibration pattern for text messages.