What are the pros/cons of buying an ETF that tracks a mutual fund, vs buying the mutual fund itself?

As an example, for an REIT ETF/Fund, would it be better to purchase VNQ or VGSIX?

you need to compare the performance. you also have to decide how you will be investing. Long Term? Size of investment?

If you will trade small amounts the commissions on the ETF's will add up. You have the advantage of being able to trade during the day with ETFs. If you are going to make regular small investments then the fund might be a better idea. Crunch the numbers to see what works.

You need to see the charts. There is virtually no difference between VNQ and VGSIX. However, there is a terrible difference between VHT(bad) and VGHCX(good).

Regretably, most people will answer this question as if fees and expenses had something to do with the answer. Dividend-adjusted return and volatility are all that matters. See www.fasttrack.net for the dividends adjusted charts

Evaluating fees and expenses is most important. The ETF will usually have slightly lower expenses a year and should go up ~0.1% a year faster than the mutual fund. (But you have to look at the expense ratios.) However, you have to pay a commission when you buy and sell the ETF. In many Vanguard Mutual Funds, you may eventually accumulate enough money to graduate to the Admiral fund, and this will further decrease your expenses.
Charting is usually not useful. VNQ and VGSIX will be so close together it will be hard to tell a difference. See link 1.
If you chart an ETF with a mutual fund and they do not track extremely closely, it means you screwed up. For instance VHT is not an ETF that tracks VGHCX, so they diverge. See link 2.