ATMs can be found throughout the country, even in small towns, though charges for withdrawls can be quite steep. In general cash is king in Vietnam, 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.