Hello there!

About the first question: currently, there are a number of  banks in Hanoi and you can exchange currency easily. I recommend for you some Banks and foreign exchange offices in the center of Hanoi and you can find one of them by looking on the map:

About the second question: credit card in Hanoi is widespread accepted in most of tourist places (hotel, restaurant,etc.). However, most of Vietnameses haven’t been use to practice credit card yet and you can’t use it to pay for taxi, street food or even ticket office. Thus, I suggest you should handle with local currency (the Vietnamese Dong) or some US dollar are also accepted.

I hope it’s helpful for you!