It's hard to beat a low-fee, broad-market index fund for long-term growth.Is the Vanguard S&P 500 ETF (VOO 0.47%) a buy now? I think it is. Let's take a closer look at the S&P 500 index and the Vanguard exchange-traded fund (ETF) that tracks it. Even Warren Buffett is a fan of the Vanguard S&P 500 ETF, writing in his 2013 letter to shareholders:One bequest provides that cash will be delivered to a trustee for my wife's benefit. ... My advice to the trustee could not be more simple: Put 10% of the cash in sh ...