The currency in Vietnam is the Vietnamese Dong (VND).
You can change your money beforehand or at leading banks in Vietnam (e. g. Vietcombank). There, you can also withdraw money at an ATM.
If you do not want to walk around carrying too much cash in your pockets, you can make use of your Visa or MasterCard, too. They are accepted in all major cities.
By the way, it is not allowed to take VND outside the country.