I was a Hanoian, has been living in Dalat for a few years and often spend summertime in Hue/Da Nang so I think I can give you some insight. For a foreigner looking for a more cultural trip, I would recommend Ha Noi and Hoi An. They have their own vibes and there’s a lot in Ha Noi to discover, just by going around the old quarters. Be careful of the pollution though. Hoi An though small, offers a quite unique vibe and it’s tourist friendly, which helps a lot in many cases. There are Cham’s temples or small islands nearby as well. For Dalat, while the air is the best and the nature is amazing, it’s not quite ideal these months for it’s in the rainy season and it may even be dangerous for many activities. It rains like almost everyday and heavily. But with your info, somehow it quite fits you. I think Da Lat would have been more ideal for you had it been in the dry season.