I went there in early February. the weather was very changeable, quite windy and we did have some rain. It wasn't pool weather even on the warmest days it was a bit overcast and windy. The rain was not like a tropical shower in a very hot climate but more like British rain that drizzled on for half a day.
Out of five days we had a day where we really needed light jackets because of the chilly wind but the rest of the time it was mild and balmy even if it wasn't especially sunny. If you are only going Hoi An and wanting pool time then you might want to move it closer to March or April but if you are planning on stopping there as part of a broader itinerary that takes in some sun in the south then PLEASE don't let the prospect of a bit of rain put you off going to Hoi An. It is the most fabulous magical place and is a 'must see' in any weather. I would love to back.