To fully enjoy your trip, remember to keep in shape once a week by walking (about 15 km) if possible on a route with difference in level (250 mph) or playing a sport (1 h at 1:30).
There are many routes for trekking in Vietnam: Sapa, Pu Luong , Mai Chau...

All your luggage is split between a backpack for walking and a second waterproof bag with padlock, transported by the logistics team on site. In Vietnam, the humidity is high in summer (between May and September) so prepare for waterproof bags contain important luggages such as cameras or other electronic equipments.

Clothing
Short sleeve shirt(s)
Long Sleeve shirt(s)
Sweater
Jeans or long light pants
Pajamas
Underwear
Socks
Belt
Jacket (lightweight rain jacket worn over sweater should be fine)
Sandals
Walking or hiking shoes (especially in the north where it might be too cool for sandals)
Bathing suit (I’m checking on whether any hotels will have pools)

Accessories
Passport
Reading materials (magazines, books)
Guidebook(s)
Maps
Cell phone – read this article about cell phone usage abroad
Cell phone charger
Camera
Camera charger
Memory card(s) for camera
Video camera (if you like)
Video camera charger (if you like)
Eye shade
Notepad/journal
Pens
Sunglasses
iPod
iPod charger
Travel documents (boarding passes, contact info, etc.)
Alarm clock
Binoculars
Flashlight
SteriPEN (bottled water will be readily available but if you want to cut down on waste, check this out)
Gifts for locals (if you like)
Postcards from your home city (to share with the locals)
Games (card or other small games)
Toiletries including:

Conditioner
Shampoo
Hairbrush
Deodorant
Hand lotion
Face lotion
Facial soap
Makeup
Razor
Toothpaste
Toothbrush
Tweezers & nail clippers
Dental floss
Feminine products
First aid kit including:
Tiger balm (for headaches)
Prescription medicines
Anti-diarrheal
Mosquito repellent
Malaria prophylaxes (consult with your doctor as to whether you want to take this)
Tea tree oil (for cuts)
Sunscreen
Antibiotic cream
Ace bandage
Handi-wipes or anti-bacterial gel
Safety pins
Eye drops or flush
Pain reliever
Lip balm
Band-aids