Have been following the different threads about Vietnam...
We are planning our trip for March starting in Hanoi, ending in HCMC. Questions:
Is 3 weeks enough/too long?
Is it best to take the train/tourist bus to make our way from N to S or is it better to fly to the different places?
Should we arrange for tours from here or when we arrive?
Are 4 or 5* hotels a must or are less expensive ones clean also?
We fly via UAE. Is it worth stopping over for 2 days?

Any other heads ups - dos - don'ts are appreciated to make this a memorable experience.
Thank you.