Clothes will depend on the weather which varies greatly from Hanoi to Saigon. Have a look on the following web site:- weather2travel.com and this will give you a good idea. I went in Jan and it was 10 degrees in Hanoi and 37 in Saigon.
With regard to washing, when you arrive at the hotel in HoiAn a lady will arrive on a scooter take your clothes away returning them the next day washed and ironed. She charges $2.50 dollars per kilo in weight. Much cheaper and better than using the hotel. I think the Tour Manager or local guide arranges this. Have a great hol it's a wonderful place.