ATMs can be found throughout Vietnam, even in small towns. In general cash is king, though credit and debit cards can be used in many hotels.
Hotels Not expected. Leave a small gratuity for cleaning staff if you like.
Restaurants Not expected; 5% to 10% in smart restaurants or if you're very satisfied. Locals don't tip.
Guides A few dollars on day trips is sufficient, more for longer trips if the service is good.
Taxis Not necessary, but a little extra is appreciated, especially at night.
Bars Never expected.