I far prefer Hoi An to a cruise in Halong Bay, and I'm also a big fan of Hanoi city itself, but I would not pack too much into a very short visit to Vietnam.

I've also been to Istanbul, and the advantage there is that all of those points of interest are close together, and transport between them is very easy. In Vietnam the opposite is true - for instance, the trip from Hanoi to Halong Bay is 4 hours one way on the road.

I recommend one or the other - If you like interesting cities and people watching then go to Hanoi. If you like creature comforts and good food/shopping - then Hoi An would be for you. You can fly directly into either Hanoi or Danang (Hoi An is only 45 minutes away).