In tax-deferred retirement accounts taxes don't matter.
ETFs are better than funds taxwise for two reasons.
1) They are more tax efficient. This means they distribute capital gains less. With a fund, if a bunch of people pull their money out, the fund has to sell stock. The sales incur capital gains, and you'll have to pay taxes even though you didn't sell anything. With ETFs you basically only have to pay capital gains taxes if you sell your own shares. Note: Vanguard ETFs are structured differently and will be as tax-efficient or inefficient as the underlying fund.
2) Tax-loss harvesting can save thousands of dollars. Here are some explanations:
http://etf.seekingalpha.com/article/15259
Essentially if you your ETFs lose value, you can sell them (and immedietely buy a similar but not the same ETF) and deduct the loss from your income taxes.