You can't know how many "people" have invested in the ETF, but you can check their website for the current fund size etc:
VAS: S&P/ASX 300 Index 0.15% 31/01/2014 513.9 million 7.2 billion
The first figure there ($513.9M) is the value of issued ETF units, the second value is the total vlaue of the underlying managed fund - remember in this case the fund existed for a while prior to availability of a listed ETF for it. Note the 0.15% figure refers to the management fees.
To track the real-time, theoretical value of a unit in the ETF, you can also use their website where they publish an intra-day "NAV" (Net Asset Value) every minute, found here:
https://www.vanguardinvestments.com.au/retail/ret/investments/etfdetailVASIFE.jsp
The NAV includes the value of any dividend entitlements that exists for the underlying shares in the fund that have not yet been distributed. Distributions (dividends) happen every three months, and include any franking credits etc that you are entitled to claim with the dividends paid by the underlying shares. You also have the option to have the distributions re-invested and issues to you as new ETF units if you want, with no brokerage cost or spread costs incurred.
You can look up an historical price chart just the same as for any other stock through a online broker, o through google or yahoo finance, the ASX website etc etc. VAS is a pretty liquid and heavily traded ETF, to the trade price history gives a pretty good historical picture. In terms of where it's likely to be, just look at historical all ords performance - that's basically what it will track pretty closely - with a small amount of under-performance due to the management fee (currently 0.15%pa).
You may also like to know that there are market makers for ETFs, who are always quoting a buy/sell offer at a reasonable spread around the NAV at any point in time. So the costs to buy/sell are pretty low. Note however that the market makers to not quote during open/closing auctions, and only start after the entire market has opened - so avoid trading these things when the MMs aren't there IMO.