Central Vietnam, like the entire country of the dragon, is divided into two distinct seasons. For this region, the rainy season from November to the end of January and the dry and sunny season the rest of the year.
Thus, from February to October, you can plan an extended stay in Hue and Hoi An with also a break to venture into the caves of Phong Nha Ke Bang National Park and enjoy the incredible nature that this area has to offer.
During the rainy season, plan cozy hotels with spas to relax after a rainy day of sightseeing. Also in terms of transport, consider having a little more flexibility, because there can sometimes be floods for a few days or typhoons for 24 to 48 hours that slow down or even prevent traffic by land or air.
Also note that at the end of April and the first days of May, if you pass by Hue you can admire a huge festival that takes place every year and is an opportunity to admire incredible musical performances and dances from around the world.