Before you leave your home country, be sure to contact your bank or card carrier to inform them when and where you'll be traveling to avoid your cards being blocked.
In major cities, most businesses accept credit cards (Visa/Mastercard/American Express/Discover). Some businesses add a 3% service fee for on top of the charge.
It is advisable to carry cash in Vietnam Dong (VND). Few touristy places accept cash in US dollars (USD) with an unfavorable exchange rate.
Note that the charges will be in Vietnam Dong so you might incur a foreign currency conversation fee from your credit card issuer on your statement.
For withdrawals at local ATMs, it is advisable to check on the daily withdrawal limit and cash withdrawal fee from your bank before you go abroad.