I am not sure how the ETFs handle changes, however my experience...

I am not sure how the ETFs handle changes, however my experience has been mixed so I think where there is a good chance a stock will enter the index the ETFs must purchase then they start buying earlier, i.e. that buying could be the reason for the magnitude of price rises more recently. Noting this is my own speculation.

With regards to trading, an ETF could be heavily traded with net outflows and therefore net sales. What you should look at is the monthly announcement on outstanding units. In the case of VAS this has grown from 80,000,000 to 100,000,000 units, or $2Bil, in the last 12 months. Therefore it is growing and will need to buy with those new funds.

I hope this helps.