Do some research on the living expenses in Vietnam to get a rough idea of how much money you need to spend there. You could divide the money into food, accommodation, transportation, and entertainment categories.
If you are on a tight budget, I would recommend you to choose those projects covered the food and accommodation. Also, knowing the distance and ways to get to the volunteer place from the dorm is quite essential in planning the budget for transportation.
You could also bring extra money for entertainment if you are going to explore the country or travel with other volunteers. Apart from that, it is safe to prepare an international card just in case you run out of budget.