Why is the VAS (ASX:VAS) share price struggling in September?

The Vanguard Australian Shares Index ETF (ASX: VAS) has been struggling over the month of September so far. Even though it's only 14 September today, already this ASX exchange-traded fund (ETF) has lost around 0.58% over the month to date.

VAS is the ASX's most popular ETF. It is run by the reputable Vanguard Group, which is not-for-profit. It currently has more than $9 billion in funds under management, and tracks the S&P/ASX 300 Index (ASX: XKO). The ASX 300 covers the 300 largest ASX shares by market capitalisation.

With such a broad slice of the Australian market, many investors use VAS as an easy way to get a piece of the entire ASX in their share portfolios.

But since VAS faithfully tracks the ASX 300, warts and all, its performance over September so far reflects that of the benchmark index itself. So it's no surprise to see that the ASX 300 is also marginally down over this month so far.

So why has VAS been struggling in September? Well, if VAS and the ASS 300 are struggling, it almost certainly means that at least some of their mutual major constituents are also struggling.

Which ASX 300 shares have been weighing VAS down?

So let's dig a little deeper. Vanguard tells us that the current top 5 shares in VAS are as follows:

1. Commonwealth Bank of Australia (ASX: CBA) with a weighting of 8.24%
2. BHP Group Ltd (ASX: BHP) with a weighting of 7.35%
3. CSL Limited (ASX: CSL) with a weighting of 6.13%
4. Westpac Banking Corp (ASX: WBC) with a weighting of 4.19%
5. and National Australia Bank Ltd (ASX: NAB) with a weighting of 3.98%

So let's see how these ASX blue chip shares have performed over September so far.

CBA shares have spent the month to date on quite a roll. This ASX bank is up 1.31% since 31 August.

In contrast, BHP has certainly had a month to forget so far. The Big Australian has lost a nasty 8.33% since the start of the month.

CSL has also had a rough start to Spring. It's down around 3.7% over September thus far.

Westpac shares have also declined the invitation to CBA's party. This ASX bank has gone backwards by roughly 0.43% since 31 August.

However, NAB shares did get an invite, it seems. NAB has managed to add around 1.51% since the start of the month.

So, it looks as though VAS investors can largely blame BHP, CSL and maybe Westpac for the struggles that this ETF has seen over September so far. But then again, we're only roughly halfway through the month, so who knows what the second half will bring for the Vanguard Australian Shares Index ETF.