QR payments here are typically only linked to Vietnamese back accounts, so I assume this allows the Chinese tourists to use their Alipay QRs to pay in Vietnam.
Yes you can always use ATMs for cash and some places take card payments, but the majority and easiest is through QR.
Source: am a foreigner in vietnam
Karen Gillen is also in that episode, before later going on to be the companion, and eventually going into the MCU.