If your plan is not staying in Vietnam for less than a month it is unlikely you will need to renew your data package. However, in a situation where you need to call or text someone locally and you do not have any credit on your phone.
Most convenience stores (including 7/11 and Vinmart for example) sell scratch-off vouchers. You just buy the desired amount and load it onto your phone. It is probably easiest to just have whoever sold you the voucher to load it onto your phone. But you can do it by yourself, you just need to open up the phone call screen on your cell phone and type *100*{voucher-code}# and press Call.