Here is a price list in Vietnam for reference (by 2015):
A bottle of water (500ml): US$ 0.3
A local beer at a bar/restaurants: US$ 0.9
A glass of coca cola in a restaurant: US$ 0.9
Pho (rice noodle soup) at a street-side booth: US$ 1
Western decent meal at local restaurant: US$12
Cash (in USD, EURO or AUD ) works in most travel destinations of Vietnam, you can exchange them to VND at most of gold shops in big cities of Vietnam. US Dollars, you can use them directly at airport and in tourist towns.
ATMs can be found throughout the country though charges for withdrawals are quite steep. You may also contact your bank for detail instructions of daily withdrawals limit and charges. Small notes will be very useful for tipping and local street shopping. If you are going to bring cash to Vietnam, please make sure notes are in pristine condition with no defects, or they will likely be rejected as counterfeit. Get the most recently dated notes you can as these stand the highest chance of being accepted.
Credit and debit cards can be used in many shops, hotels and decent restaurants.
As to tour cost, you may compare