An increasing number of Hanoi outlets accept MasterCard and Visa, but the capital city is still a cash-based economy. American Express is accepted less widely than other major credit cards. Note that most businesses will charge you an extra 3% per transaction. Look for credit card logos pasted to the inside windows of participating merchants' shops. If in doubt, ask before assuming your credit card will be honoured, especially if you're only carrying AmEx.
Although more and more Hanoi merchants are accepting credit cards it is advisable to also carry cash, especially at places where payment policy is unknown or vague; after all, nobody wants to lose face come payment time! It's pretty much guaranteed that most Hanoi luxury hotels will accept major credit cards such as Visa and MasterCard. Some establishments will charge a higher user fee for American Express holders. It's recommended to ask before assuming that your credit card will be honoured.