Hi Kingma, I've been facing the same question and done some research about it during the last couple of months. The thing is you're actually looking at slightly different products:
What you buy through your broker Commsec is shares in Vanguard ETF, which have a very low expense ratio (I think 0.15% for VAS which tracks ASX300, 0.05% for VTS which tracks the US index) but incur transaction fees.
When you open an account with Vanguard you buy in their managed fund, which don't have transaction fee, but a much higher expense ratio: 0.75% for the first 50k for the Australian share fund.
So you get more flexibility with the ETF, and potentially lower cost if your transaction are big enough to make the fee small in comparison. If you buy the fund there are minimum amount and Vanguard takes care of all the tax reporting, allows you to transfer automatically with BPay, but it's more expensive when you have a significant amount.
You'll have to do the math depending on how much you can invest each time to see which one ends up being cheaper. The good thing about the fund is that you can put money in their Lifestyle funds, which set and keep an allocation for you. For instance the High Growth has 40% Australian shares, 40% International Shares, 10% property and 10% bonds. That makes it easier as you don't have to keep track of your allocation and rebalance, but obviously this come at the price of the bigger expense ratio.
We unfortunately don't have the extensive services and lower prices available in the U.S., but hopefully it'll get better over time.
Cheers