The onboard currency for the diamond princess will be american dollars, however when you on shore you will need to have the currency that the country uses.
Can you use american traverlers cheques in stead of money on boat. how much did you change for each country.
Onboard currency is US Dollars but also any Princess transfers from ship to town will also be US dollars and will need to be paid in cash. Although it is best to have local currencies, US dollars are widely accepted in Vietnam.