Investors love the Vanguard Australian Shares Index ETF (ASX: VAS) for a number of reasons. After all, it's not usually just one perk that leads an index fund like VAS to the top of the ASX pile in terms of funds under management.

The Vanguard Australian Shares ETF offers investors many advantages, including instant diversification, exposure to a broad range of quality businesses and historically decent capital growth. But one of the largest perks is arguably this index fund's dividend potential.

The VAS exchange-traded fund (ETF) tracks the S&P/ASX 300 Index (ASX: XKO), which means it invests in the largest 300 shares on the ASX by market capitalisation (size). That's everything from Rio Tinto Ltd (ASX: RIO) and ANZ Group Holdings Ltd (ASX: ANZ) to Telstra Group Ltd (ASX: TLS) and Harvey Norman Holdings Limited (ASX: HVN).

The top echelons of the ASX are jam-packed with dividend payers, as you can see above.

This means that the VAS ETF is also jam-packed with those same income-spewing stocks.

In addition to the companies listed above, the VAS ETF also holds and receives dividends from Commonwealth Bank of Australia (ASX: CBA), Westpac Banking Corp (ASX: WBC), Woolworths Group Ltd (ASX: WOW) and Fortescue Ltd (ASX: FMG).

But let's get to the crux of the matter, and work out exactly how much in dividends investors enjoy from the Vanguard Australian Shares ETF.

As an ETF, VAS can only pay out what it receives in dividend payments from its underlying holdings. As such, the quarterly dividend distributions from the fund tend to fluctuate, depending on what kinds of dividends its constituents are paying out.

So over the past 12 months, VAS has forked out four ASX dividend distributions. The distribution for the March quarter (paid out on 17 April) was worth 84.79 cents per unit. The December quarter payment preceding it was worth 71.62 cents per unit. The September quarter payment came in at $1.29 per unit, while the June quarter's distribution was worth 88.9 cents per unit.

That works out to be a trailing annual total of $3.74 per unit.

At the current VAS unit price of $95.59 (at the time of writing), this gives the ASX ETF a trailing dividend distribution of 3.91%.

Since many of VAS' underlying ASX holdings pay out franked dividends, VAS' own distributions usually come partially franked as well. This also tends to fluctuate from quarter to quarter. But the most recent payment came partially franked at 78.2%.

So that's the kind of dividend yield investors have enjoyed on their Vanguard Australian Shares ETF units over the past 12 months. Not a bad yield for a diversified index fund, one could argue. Let's see what kind of income investors enjoy over the coming 12 months.