Vanguard is the perfect place to compare ETF and OEF, because they invest in the same underlying portfolio and their shares, say VIG and VDADX, and have identical ERs. This removes portfolio differences and ER differences, allowing one to see clearly the differences due strictly to the sales structure. It also demonstrates that with Vanguard, cost advantage is indeed illusory (#1).
With VDADX, you get exactly what you pay for (NAV). With the VIG ETF you may get more or less, and that difference varies from moment to moment and day to day (#
5). Vanguard shows that as of now (market close, Oct 6) the ETF was trading at a $0.04 discount, so selling near day end would have netted you a few cents under NAV.
Actually, you likely would have lost another penny, because Vanguard (like most providers) gives the closing price of the ETF as the midpoint between bid and ask. VIG typically has a 2c spread, and depending on the Vanguard ETF, the
typical spread can be as high as 0.20%. (#4).
Hidden expenses, Vanguard? Trading expenses - these are the same for VIG and its open end siblings, because they share the same underlying portfolio. Likewise any volatility due to skittish investors (again one must ask, Vanguard?) would impact the NAV of the ETF share the same as it would affect the open end class' NAVs.
This is why I feel Vanguard provides the perfect way to compare the two ways of buying/selling shares of a portfolio - ETFs and open end funds. Take the portfolio itself out of the equation and all that's left are the intrinsic differences in sales mechanisms. One always gives you a buck for a buck (NAV), the other often doesn't quite get there, for various reasons.