I did a cruise, on the Mekong, from Siem Rep (or nearby) to Saigon (whatever it is called nowadays) about 5/6 years ago.
I was advised to and took USD in new singles and nothing bigger than 20$.
They were accepted everywhere.
I only used my credit card to pay for hotel bills (Marriott if I remember correctly).
Maybe I am overcautious but I would not use a card elsewhere.