Are there many ATMs in Hanoi? Are here any at the airport?

Yes.
Yes.

lots

Yes, there are quite many ATMs in Hanoi, by the way, you can go to Ha Trung Street where you can change a lot of currencies into Dong with good rate.

Be very careful. After years of travel I finally got pickpocketed in the elevator of the Metropole in Hanoi. Withdrew dong from the Citibank ATM right outside the hotel and went straight to the room, taking the elevator with a scruffy looking gent. No wallet once I reached the room. Hotel staff searched the room as did I. Gone. Enjoy Hanoi!