Started in Hanoi and bought a scooter for about $300 USD then the two of us spent the next few weeks exploring off the beaten path in North Vietnam. No Ha Giang or Cao Bang Loops; in fact no defined route at all. Just talked with locals and searched around on Google Earth/Maps. Aside from a few of the more popular towns, we rarely saw other foreign tourists.
Can't say enough good things about Vietnam! Between the incredible nature, delicious food, and extremely helpful and welcoming locals it may be our favorite destination.