I'd like to recommend Rentabike Vietnam , me and my friends rent four Honda XR150s to spend over two weeks on motorbike trip.
We spend amazing time in journey from north to south , our trip has based on - The best North to South routes in Vietnam Ultimate Route maps but we add and we've did also Ha Giang loops.
The maps are available on Rentabike website , are very helpfull whilst the journey planning.
The Rentabike staff in Hanoi is very friendly,professional and helpful.
Our trip has ended in Ho Chi Minh , RAB staff pick up our bikes directly from our hotel , it was very convenient for us.
All our bikes worked very good during our amazing journey , we've not any problems, so I can also recommend Honda XR150s.
I can honestly say , if you are thinking about renting any bikes to spend amazing time in beautiful Vietnam , go straight to RAB :):):)