Need tips for trip to Vietnam

Do lots of shopping on your first trip. Everything is cheap. There is nothing you will buy that you will regret later. Negotiate, negotiate, negotiate. Very few stores have fixed prices or anything that is unique. Do not be afraid to be firm about your top price and walk away. You will find the item someplace else if the vendor does not then meet your price.
Taxis are VERY inexpensive, readily available and metered (i.e. no negotiating necessary). We stayed up near the zoo and the fare plus tip all the way to Ben Thanh market was never more than $1.50.
Visit Ben Thanh (or any other local market) early in the morning when vendors are setting up. We arrived at 6am and could not believe the bustling activity. It was very exciting and a whole different experience than walking through the market at midday.