I was there last year. HCM City is good, a lot of history to the city. The big tower building there is great to have a drink at dusk and take in the views. Hanoi is absolutely mental, thousands of motorbikes on the streets. Both cities are good for a few days. Halong bay was a big dissapointment for me. It was misty, overcast and not the warmest when I was there. It resembled Cleethorpes beach on a winters day. Not a great choice of accommodation in Halong Bay too. I only had 14 days on my visa so I didn’t get the chance to do the coast. Best thing about Vietnam is the beers. Think the cheapest beer I managed to get hold of in one bar was about 20p. Nice beers too, 333 I think it was called. The weather is a bit cold in the north in March, so just be careful and check what the temperatures are like before you book, if the suns one of the things you’re after.