I actually asked a similar question on this forum about a week ago but have not had any response yet. Perhaps keep an eye on the thread: https://geoexpat.com/forum/155/thread355062.html
I don't have an AUM number in mind but there have been questions, including on geoexpat, about 3140's liquidity. However, this and other Vanguard ETFs have grown increasingly popular in the last few years so I would expect the liquidity to go up over time. I don't think it is likely that Vanguard will close these ETFs due to lack of AUM anytime soon, but who knows what could happen with a 30-40 year horizon. It's something to keep in mind as a relative risk. Someone else on this thread mentioned that Vanguard has closed ETFs in the past in HK, although I can't find any info on this. BlackRock has definitely closed down ETFs in the past in HK.
Bear in mind that the smaller the AUM, the harder it may be to liquidate your portfolio. Up to you, but you may want to also include 1-2 other broad ETFs in your portfolio with decent liquidity (2800 ETF comes to mind, with the added bonus that you can set up automated investing into it every month with your bank) to counter the risk. I started buying 2800 ETF through automated investing via my HSBC account recently. The AUM under 2800 is huge and you won't have any problems later on with liquidation, whilst the fees on this ETF are even lower than Vanguard's.