You don't need american dollars or travellers cheques (the latter are just a pain). There are plenty of cashpoints and I found the local banks cashpoints give a better exchange rate than eg HSBC.
Plan carefully what you want made in Hoi An and take a picture if you can - you can get pretty much any clothes and shoes made in virtually any fabric, but you don't actually get much time to decide and order.
Don't need to take a hat - contrary to the notes, they are easy to buy there.
Water is a must, but available to buy everywhere. Cheapest we got was 8000 for 1 1/2 litres.
Haggle, but know when not to - your guide will help.
Khong cam on (no thank you) works better in Vietnamese than English - you will get pestered.
Be prepared to spend a lot of time on buses plus 10 hours on the train. You are seeing the whole country and its huge so there is no getting away from it.
Take an overnight bag for Ha Long Bay and the Mekong Delta (go and see the crocodiles :-)