Followings are the few things are my top investment before my trip:
1. camera SD card reader to Iphone
2. selfie stick with three brightness adjustment mode
3. airplane footrest:
4. neoprene Camera Case for my camera
5. Folding travel bag ( you can use it on the way back if you have more things to carry. The bag can collapse down to a small size and takes up almost no space when not in use)
I know that we will be exposed to enough sun, I planed to purchase a hat before. But Bao told me the other day that leaf hat is one of traditional things in Vietnam. I guess I can buy it there.
Also, the waterproof packing cube is always my favorite thing to pack my luggage. Here is just one link for your reference: https://www.amazon.com/HiDay-Set-Packing-Cube-System/dp/B01F8OZV3S/ref=sr_1_11?s=apparel&ie=UTF8&qid=1490804932&sr=1-11&nodeID=7141123011&psd=1&keywords=packing%2Bbags&th=1