Hello Ashi, I had been to Vietnam last April. One easy way is carry US dollars to Vietnam and exchange to VND at the airport itself. there will be very less fee. Happy travelling and get used to counting in Million system 😀
Though the official currency of Vietnam is Dong (VND), many big shops and hotels of Vietnam accept US dollar as well. When you travel to Vietnam, it's handy if you bring some dollars.
I had the similar experience during my visit to Cambodia in Dec 2018 . So carry USD in different denominations . You can buy these USD in India itself.
Asfar for VND , you can buy the same in Vietnam itself .
Happy Travelling !!