Hanoi (in the North) is colder at night and much-much sunnier. But Central Vietnam is open to the huge South China Sea and gets all the precipitation from there. That's why the temperature at night and afternoon is almost the same: it's because Central Vietnam has so clear sea type of climate (maritime climate) that... itself "is" the sea.
And the temperature of the seawater (surface, by the shore) is 22.
Basically if it wasn't rainy, you could spend whole winter at the sea shore, 22 is good temp, not cold not hot