Not. You can choose your own denominations of 10,000 VND, 20,000 VND, 30,000 VND... It is not necessary to choose the same denominations of Vietnam currency (10,000 VND, 20,000 VND, 50,000 VND, 100,000 VND...)
In addition, the vouchers purchased at Got It can be split in value, the gift recipient can split to use the last value.