Hoi An is really wonderful, and I know it quite well after 3 trips (7 trips to Vietnam). I was last there in March '15, and stayed at a wonderful little hotel (Riverside Impression Homestay). I'm not a big fan at all of Hue, and unless you're a history buff, you might not like it either. Your problem with the north central coast could be weather in Jan - could be cooler and overcast. I usually go in March or November when it's wonderful. I don't care for Nha Trang at all - beach is good, but I really hate that city (no good food for one thing!)
You might not have considered Quy Nhon - it's the sleeper of Vietnam (off the tourist trail) and wonderful. This small city is ultra-friendly to Americans and probably the cheapest place on the coast, but very little English spoken there. I loved it, and will be returning. You might look at a little place a bit further south in the village of Bai Xep called Haven Vietnam - aka paradise lost! Quy Nhon has VN airline service from SGN and HAN.
Luang Prabang is wonderful/beautiful, but for such a short trip it would be rather expensive (over $300. roundtrip from Hanoi). LP is always warm year round - I've been there twice.
Last edited by Daawgon; Jul 1, 2015 at 11:04 am