Hi Julia
You are quite right about the weather in Vietnam, it does vary from area to area. I was on the same trip as Dave. It is certainly cooler in the north in Hanoi and much hotter in the south Ho Chi Minh city. November was the coolest time, if you could call it that to visit these two areas, according to our guide book. BUT it was the worst time of the year to visit the central area i.e Hue and Hoi An because it was the wettest time there. They have typhoons ranging from minor to mega, landslides and floods. So Dave and our other friends before we went were joking about taking dinghies, inflatable armbands and rubber ducks etc etc. However, although it did rain and heavily at times it was always overnight, whilst we were on a coach or in a reataurant. Didn't bother us!
Super holiday. My highlights were Halong Bay, Hoi An and the cycle ride. I was dubious about the cycle ride because I am not the fittest person in the world but it was easy didn't actually cycle far, lots of photo stops, rode mainly on paths through paddy fields. Much fun when some of the group decided they wanted to ride on a waterbuffalo.
Happy travelling and kind regards Di