Overnight trains between cities. They are cheap, reliable and adequately comfortable. Compartments have four or six bunks. The doors are locked from outside (but you can open from them inside). You will are as likely to be travelling with locals as other Western travellers.
The Open bus which travels from Saigon to Hanoi and back and stops at at numerous cities in between is cheaper than the train but less pleasant. The seats are permanently reclined and claustrophobic. It is largely used by Western travellers.
Xe-om (motorcyle taxis) are a convenient and often unavoidable way to get round town. The drivers are very pushy and often dishonest. Quite a few seem to double as drug dealers and/or pimps.
Bogus taxis work the airport at Hanoi. The drivers approach you in the terminal. At best, they will overcharge you. That said the regular taxis have a poor enough reputation.
Cyclos (cycle rickshaws) are a fun way of getting round Saigon and Hanoi, but some of the riders pull all the same scams as the xe-om drivers.
Scams. Xe-om and cyclo owners alike famously quote 15,000 Dong then insist they said 50,000. Get them to write it down, or write it down yourself and get them to confirm. (Keep in mind, though, that the sums are small and the cyclo owners in particular probably need the money more than you
Taxi drivers in Hanoi have been known to take travellers to clone hotels (which have ripped off the name of a popular hotel) or to insist that their hotel has closed and suggest an alternative.
Need to book in advance?
Open Bus, yes. Trains, generally not.
Widely among the people you will deal with.
Vietnam, and particularly Hanoi,is full of small-scale scammers and vendors who will not listen to “no”. But neither should deter you from going or spoil your visit.