I just came back on March 27th from 18 days in Hanoi, Quy Nhon and Saigon. March is always good, but this year Hanoi had quite a bit of light drizzle and rain in March, but the central coast (Quy Nhon was quite beautiful and mild) was a different story. Saigon is always blazing hot. April is the beginning of the real hot weather everywhere in Vietnam. Another good period for the area is late Nov-Dec - the extreme weather is usually over by then on the central coast.

There is no question but Hoi An is the better option on the central coast, in my opinion, but it is touristy. Since I've been to Hoi An several times, I decided to try Quy Nhon for this trip, and was pleasantly surprised. Basically, very few tourists in sight, a very clean and orderly little city with great and inexpensive seafood. Quy Nhon doesn't have the attractions that Hoi An has, and there is little shopping that would interest westerners, but I loved it for a few days. You should know that once you get off the tourist trail (Quy Nhon is well off this trail) English comprehension is unknown, but you will do well there without English - just don't expect perfection.

I avoid Jan-Feb in Vietnam due to higher prices and the problems of TET - in the north, it will also be pretty cool and rainy at that time.