Picking up a taxi or sharing a van on-spec at the port works fine at minor ports in Vietnam, but not the two main ports.
Saigon - if your ship is berthed in Vung Tau or Phu My you'll need to pre-book either a private or ship's tour or ship's "Saigon on your own" transfer, since the city is about 2 hours away. Do consider booking an overnite city hotel. If your ship berths in the city (small ships such as Azamara) you can pick up a taxi or (Nha Rong port) its a 15 min walk to the centre, and there are plenty of (much cheaper) liveried metered taxis for the return. Once in the city centre, the main sights are very walkable subject to just a little research.
Halong Bay - if you want to go to Hanoi, again pre-book a private tour or ship's tour, the city is about 3 hours away. If you want to take a junk cruise in Halong Bay, you can fix this up at the pier. Its not worth taking a taxi tour of Halong City or bay, we found little of interest on the mainland & most drivers don't speak english.
Bangkok - if you berth at Laem Chabang, its about 90 mins from the city & a cartel operates the taxi rank in the port - with rip-off prices. Best to pre-book a private tour or transfer, or ship's tour or transfer. With a little research you don't need a tour, the city is easy to get around by the Skytrain, express ferries, taxis, tuk-tuks. As with Saigon, consider a city hotel if your ship overnites.
Just MHO.
JB