I spent a week there back in 2001. Crazy! cool! unlike any place else I have ever seen.
You will not have a problem finding people who speak English or want to talk to you. There is at least one HSBC in the main square in Saigon where American ATM cards will work.
I would caution you about taking the buses. Drivers there tend to play chicken a lot with oncoming traffic because the roads get clogged with all the bicycles, animals, people on foot, etc. on the shoulders. At least, don't sit on the drivers side of the bus. A fun road game is "How many children fit between mommy and daddy on the moped?" Most I counted was 4 but they can get more than that.
I did the day-long Mekong delta trip and it was roughing it about as much as I ever have. 3-days may be a little long, by that point in your trip. There is a lot to explore in Saigon, I would not rush through it. Don't rush the beach either. beaches are gorgeous! I forget the name of the town where we stayed. Eat lots of fresh seafood with lime juice and salt and pepper.
Don't miss the Caodai temple/cu chi tunnels day trip, its a couple hours by bus from Saigon. It was amazing.
Another fun game at the market is "How many different types of dead animals are packed into that bottle of snake liquor?" You will see what I mean! Men may encourage Bill to drink it. I've had it, it won't make you sick but it is definitely the most disgusting alcoholic beverage I've ever had. Pretty cool to say you drank out of a bottle with a dead cobra in it, though.
Trish explored more of the country and can tell you guys about Cambodia and Singapore too. I'll ask her to post.