Agree with #2. I think a day too long in LP. Halong Bay is not worth considering unless you are going to do an overnighter, normally on a boat. So you'd need to block out 2 days for that including return road transport from/to Hanoi.

For this exact sequence, you will need a double-entry visa for Vietnam of course, or two separate prearranged VOA's. In addition to that, it might be cheaper if you either front- or back-loaded all the Vietnam into a continuous sequence and traveled SGN-HAN or vice versa by domestic flight. Those regional international Indochina flights are annoyingly expensive given their short duration. Unless you can find some sort of airpass that works for you and is represents good value--easier said than done.