As noted in previous Deal posts, free Sing$20/($40 China only) vouchers are currently available to eligible passengers transiting through Changi Airport.
The deals are for:
Qantas flights https://www.ozbargain.com.au/node/482590 which expires on 31 December 2019; and
Singapore Airlines/SilkAir/Air NZ (some only) flights https://www.ozbargain.com.au/node/447967 which expires in 2 days ie 30 September 2019
However I have noticed that the Singapore Airlines Vietnam site https://www.singaporeair.com/en_UK/vn/plan-travel/local-prom… is showing that the scheme is open until 31 March 2020. It doesn't specify flights to/from Vietnam. However, it does state "Other terms and conditions apply."
It may be worth (otherwise eligible & non-Vietnam) SQ travelers flying after 30 September fronting up to the iShopChangi Collection Centres to see if they can get a voucher.
And reporting back here, of course - as I will be traveling in the middle of October. :)
Please do let us know. I will be transiting through Singapore in Nov. Would be nice to still have it then. Going to check out the giant indoor waterfall. :)