August has been a good month for the VAS (ASX:VAS) share price

The Vanguard Australian Shares Index ETF (ASX: VAS) has had a remarkable month of August so far (touch wood). This popular ASX exchange-traded fund (ETF) mirrors the S&P/ASX 300 Index (ASX: XKO). As such, if VAS has had a good month, it means the ASX 300 has too. 

But let's get into some numbers.

So over the month of August to date, the ASX 300 Index has climbed from around 7,386.5 points at the start of the month to 7,538.8 points (where it stands at the time of writing. That's a gain of roughly 2.1% for the month to date.

Meanwhile, the VAS ETF started August at $94.56 a unit. Today, it stands at $97.01 a unit at the time of writing. That's also a gain of roughly 2.55%. So VAS has slightly outperformed the index that it tracks over August so far. This is probably down to some quirks with index allocations. But putting that aside, it's been an indisputably good month for both the ASX 300 and the Vanguard Australian Shares Index ETF.

But why?

Well, like any ETF, VAS's performance is directly correlated to the performance of the underlying shares it holds. And since VAS mirrors the ASX 300 Index, VAS will hold the same shares, in the same weightings.

As you might guess, it's top 6 holdings, in order, consist of the big four banks, BHP Group Ltd (ASX: BHP) and CSL Limited (ASX: CSL). Rounding out the top 10 are Wesfarmers Ltd (ASX: WES), Macquarie Group Ltd (ASX: MQG), Rio Tinto Limited (ASX: RIO) and Woolworths Group Ltd (ASX: WOW).

Which ASX 300 shares have fuelled VAS's rise over August?

Well, we can rule out BHP and Rio for being responsible for VAS' August gains. Both of these iron ore miners have had a terrible month. This is partly the result of lower iron ore pricing over August.

CBA has been pretty modest, giving investors a 0.56% gain over August. That's very similar to the returns of Australia and New Zealand Banking Group Ltd (ASX: ANZ). But it's the other ASX banks that have shone inside the VAS ETF. Westpac Banking Corp (ASX: WBC) has put on a very healthy 5% over August so far. National Australia Bank Ltd. (ASX: NAB) has done even better, gaining around 6.4%.

CSL, however, has been the star of the ASX 300 over the month about to pass. This health care giant has managed to gain around 9% since the end of July.

So the numbers are in. Investors can largely thank CSL, NAB and Westpac for the stellar month both the ASX 300 and the VAS ETF have enjoyed.