If you wish to diversify across index funds, which I think is a wise idea (not to improve their returns, but to avoid the risk that the company running the fund could collapse), I suggest an excellent alternative might be the SPDR S&P/ASX200 fund, which is an ASX-listed index fund with the code STW.
The advantage of STW over the Vanguard fund is that STW's fees are considerably lower, which translates to better returns over the long run.
Hopefully as people begin to realise that the index fund is undoubtedly the smartest way to invest, more companies will create index funds. This would create a competitive marketplace, reduce fees, and allow us to diversify risk across additional companies.
But for now, it's Vanguard and STW...