I think you can visit Hanoi and HCMC within 6 days. You can take a flight of Vietjet or Vietnam Airline. Both of them are reliable. You can enjoy street food in Hanoi Old Quarter. Ben Thanh market (HCMC) also has many business activities and good food. For hotels, in Hanoi, you should rent a hotel in Old Quarter, for example, Hang Trong. In HCMC, if you want to stay at 4-5 star hotel, you can consider Novotel, New World, Sofitel or Caravelle.