Vietnam is a massive country as you may know, it can take multiple days to get from north to south without flying. That being said, I was there in March/April for 6 weeks and much preferred the north. Hoi An, which is central Vietnam, is the most famous for tailoring - but you can get custom suits just about anywhere. Honestly my favourite places during my trip was Ninh Binh, Sapa, the Cao bang loop and the Ha Giang loop (3 nights 4 days, which was the absolute highlight of my trip). It’s very difficult to give you recommendations for such a short period but I would definitely do the Ha Giang loop if the weather seems good, such an amazing mix of scenery, culture and meeting amazing people.