Which decentralized, non-custodial exchanges are used by Vietnamese to trade BTC<>VND online?
There is no decentralized and non-custodial exchanges to directly trade BTC to VND, as far as I know of. It is already difficult to find truly decentralized exchanges so that having one at least to trade BTC to local fiat is zero.
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All in all, they are not decentralized and not non-custodial so that you can not have full control of your Bitcoin wallets and your funds. Your funds are on the hands of exchange/ marketplace owners and this is a big risks. They can scam you or whatever to steal your funds.
Thanks.
Given your answer, why then don't the VND market makers at those exchanges create liquidity at Bisq exchange?
It is decentralized and non-custodial, and has been available for years.