Sapa has no single best month, only months that suit different things. Gold terraces in late September, mirrored water fields in May, cloud seas through the winter, and a rainy stretch that turns the trekking paths to clay.
Ninh Binh is open all year, but the difference between June and October is enormous when you are sitting in an open boat for three hours. This is what each stretch of the year is actually like, including the short rice harvest window everyone chases.
Hanoi is one of the few places in Vietnam with four genuinely distinct seasons, and the difference between them is wide enough to change your trip. Here is what each one is actually like, and which window is worth planning around.
Ha Long Bay is a seascape, so weather changes the trip more than it would inland. Autumn brings the calmest water and clearest air, early summer is bright but crowded, and July to August is storm season when boats get canceled.
Late September through November is the strongest window for Ha Giang: dry roads, clear air, gold rice terraces and then the buckwheat flowers. Here is what each season actually looks like up on the plateau, and which months are worth thinking twice about.
Cat Ba is busiest exactly when its weather is least reliable. This breaks the year into four honest stretches, with what each one gives you and what it costs you.