Any tips , tricks, must do's??

Stay there - it's f' awesome.

Been to all 3 several times and Hanoi is the favourite, go to the Maison Centrale, the infamous "Hanoi Hilton" prison as well as the military museum and a cyclo ride around the old quarter is a must, get your hotel to arrange it so you don't get ripped off.
When getting taxis in Vietnam make sure you get the Mai Linh taxis as the meters actually work, they are green and white.
We took a Halong Bay cruise on the Halong Ginger, expensive but very nice, the drive down there is a bit manic with the traffic.
HCMC has a much faster pace with more cars and the airport is not too far from town but it takes ages due to the traffic.
We went to the history museum which was very interesting as well as the Cu Chi tunnels which are well worth a look, plan on no less than 3 full days in each place.
Hoi An old town near Da Nang in the centre of the country is a beautiful place for a relaxing few days.
Use your staff travel links for places with an industry discount or PM me.

In Halong Bay I was very impressed by the sight of a collection of liquid water droplets suspended in the air hugging the Earth's surface. Didn't see anything else though.