for me, they asked for 1) proof of source of income and 2) a photo of every passport page with a device showing current date and time (could be to check whether I am still in Vietnam).
Anonymous participant 645 for further docs, they asked for every page of my passport. You can get another device like a phone or laptop (I used my iPad) which is showing current date and time and take a photo of every page of your passport beside that device.
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