By the way, a bike from Cuong or Flamingo should be fine, but it should only take one glance at the descriptions of bikes being sold by travellers via the Internet (invariably, something like "engine rebuilt only 500km ago!!", as though that's a desirable thing) to understand the general state of the motorcycle fleet in Vietnam. In Hanoi and HCMC, actual motorcycles (as distinct from scooters) are generally only bought and sold by foreign tourists, so they're in about the worst possible condition imaginable and - at that - Chinese replicas of bikes in the worst possible condition imaginable.
One further tip: bring saddlebags. I'm considering another trip to Vietnam (the first was interrupted by gastro) and I've yet to hear of anywhere that sells reliable gear.