It depends well or the season, and the area you are going to trek. It will says in the itinerary if the village has vehicle access or the trekkers are totally independent of a vehicle. Hereuner is some basic guidelines: when packing for a trek
Shoes; a good pair of running shoes are perfect, or even better are the lightweight trekking boots, light pant.Boat trip in Mekong River
Absolutely essential are sun protection (sun lotion as well as a sun hat.) and mosquito repellent. Mosquito nets are provided in the lodges.
Warm sweater or pullover during the cold season (October/November to February)
Special medication. Hope it will hepl!