I too would say March onwards, but be warned that early March can be cold in the north - this year it didn't warm up in Hanoi until about the third week of that month and it was often overcast and drizzly.
Out of 4 trips, I would have to say March, then Nov-Dec (I don't like it really hot). They say that April is the hottest.
in nha trang, july and august are the hottest
When you say hot or cold what temps are you referring to as cold to some people is 15c where hot could be above 30c.. Hate to be more specific but could you mean mild and the temp is actually 20-25c ...So confused by what you call as hot and cold Cheers macca
Because of the very high humidity in Hanoi and places nearby, the damp air makes 15 C there feel much colder than the same temperature does in other places.
For example, when I arrived in Hanoi in March this year and the temperature ranged between 15 and 20 C for the first 2 weeks or so, I felt far colder than I had at 10 C in the UK, where the humidity is lower - about the same as in Melbourne, if I remember correctly.
I didn't start to feel warm until the temperature climbed to 25-27 C, whereas in the UK I would be feeling very hot in the same temperature range. A little over 30 C can be considered hottish, and nearing 40C it's debilitatingly hot, but that doesn't happen until mid to late April in the north.
summer in nha trang is mid to high 30's'c, humidity is not too bad, being by the sea is not too hot
cold in december, with the winds and humidity, it gets down to +20'C, maybe 18'C at night, in the daytime it may get down to 22 or so, feels quite cold
I was there in July and feel the weather is great with sunny at day and it's blowing very coolly at afternoon.