Please note that Vietnam is a developing country and size selection and/or quality products are real issues. Local products are cheap but almost just one size for all or the quality is poor. Here are some suggestions:

-Clothing: helmet, rain gloves, wellington boots (rubber boots for rainy season from April to September), summer gloves, balaclava, t-shirt, socks & underwear, long sleeved shirt, turtleneck shirt, extra jeans, light jacket, leather chaps, zip-lock bags, riding boots, bandannas, sunglasses and goggles.
-Personal items: basic toiletries, emergency cash, sunscreen and earplugs.
-Emergency items: first aid kit, emergency contact number, list medical conditions, list medications, flashlight, chargers and duct tape.
-Miscellaneous: small towel, bath towel, trash bags, bungee cords, camera and cargo net.