Can I use US dollars or other foreign currencies in Vietnam?

It depends, you can use US dollars / Euros/ English pounds/ in the hotels to pay for the bill, in the Duty free shops, to buy things from those vendors outside the scenic spots, to pay for any optional tours and to tip porters of the hotels, tour guides and drivers or whomever you want to tip. While you can not use US dollars or some other foreign currencies in department stores or small convenience stores, local restaurants, or if you try to buy things from street vendors.