I visited Vietnam Last year, I am not sure about the legality of Bitcoin in here, but you can't directly purchase anything with Bitcoin as far as I know. There is a Vietnam based crypto exchange
https://bitcoinvn.io/ so I guess it's not completely illegal, but to use this exchange you will need to have account on Vietnam local bank.
My advice if you still want to use Bitcoin as your main reserve is you can ask for Debit Card from your bank, then use your usual way to convert Bitcoin to your own local currency and then transfer it to your bank account, and use your debit card to withdraw some cash in Vietnam ATM.
That's what I did on my last visit to vietnam
