The India side: the whole gain at 12.5%
India taxes the full capital gain from your original cost. For long-term property sold on or after 23 July 2024, the rate is 12.5% without indexation, and as an NRI you get neither the resident option of 20% with indexation nor any currency relief for the rupee's fall, so the whole rupee gain is taxed. The buyer must deduct TDS under Section 195 on the sale, plus surcharge and cess, not the small 1% that applies to a resident seller, so a lower-deduction certificate is worth getting and any excess is reclaimed by filing an Indian return.
The point NRIs get wrong: the choice residents have, 20% with indexation or 12.5% without, on property bought before 23 July 2024, is for resident individuals only. An NRI does not get it and pays the flat 12.5% without indexation, whatever the age of the property.
Vietnam's tax is smaller, so the credit clears it
Vietnam taxes a real-estate sale by an individual at 2% of the sale price. Note that this is 2% of the whole price, not of the gain, so it is charged on the full sale value. Even so, it is almost always far smaller than India's tax, because India charges 12.5% of your gain, and on a property that has risen in value the gain-based 12.5% comfortably exceeds 2% of the price.
Because India, as the country where the property sits, has the primary right to tax the gain, Vietnam gives a credit for the India tax against its own charge. Since the India tax is the larger of the two, that credit wipes out the Vietnamese 2% entirely, and Vietnam collects nothing extra. So the practical outcome is clean: India's 12.5% is the real and only cost of the sale, and the Vietnamese tax, while it technically applies, is fully relieved by the credit. A practising CA computes the Indian gain, gets the lower-deduction certificate so the buyer withholds on the real gain, and gives your Vietnamese accountant the India-tax-paid detail for the credit.