We went to Vietnam in July/August 2018. It was great! you'll have a ball, and lovely food! It was hot and humid, but not as bad as, say, Singapore, so keep the clothes cotton/linen, thin and light.
We went to Ho Chi Minh - great restaurants and there was a night market/food market with lots of street food stalls, cheap beer and open tables, with raucous live music, which was a lot of fun. We took a day trip on the Mekong which involved biking across the islands visiting various micro-businesses, a river trip and a delicious lunch on board. We took another trip by boat to the tunnel complex where the Viet Cong had a stronghold. You can fire AK47s at a range there and again, a brilliant lunch is included. The city itself is very exciting - the People's Palace is a beautiful mid-century modern building set in a pretty park, the history of the war is a great exhibition. We didn't do the food tour by scooter, but that looks like great fun.
Hoi An is lovely, although I was disappointed in the 'tailors'. I had visions of getting beautiful copies of Chloe shirts but they seemed mostly devoted to kitting out Australian families in identikit fruit-themed shorts and shirts co-ords. My in-laws did a tour of the main market and a cooking lesson when they went a few months later, which they loved. We stayed in a beach resort for this phase and got a shuttle bus into Hoi An, so this was more of the chilled, swim in the sea, get a massage phase of the holiday for us before we flew to Hong Kong.
Back to the clothing: by the time I'd spent two weeks washing and wearing the thin light capsule wardrobe I'd organised for Vietnam, I was feeling a bit scruffy in Hong Kong. So I bought a couple more items at that point. So if you think that might be you, maybe take an outfit that is more city restaurant/fancy shopping centre appropriate.