The Vanguard S&P 500 ETF (NYSEMKT: VOO) is not only a massive exchange-traded fund (ETF) -- it's one of the largest investment vehicles in the entire market. Many investors consider the ETF to be the backbone of their portfolio, and for good reason. The S&P 500 has historically delivered excellent results over long periods, and is generally a worry-free way to invest over the long run.

However, with the S&P 500 near an all-time high, many investors are wondering if now is the right time to invest. Here's what you need to know, and three important things to keep in mind before you buy shares of the ETF.

It's common knowledge that one of the main goals of investing is to "buy low and sell high," and with the S&P 500 close to its all-time high, it might feel like you're doing the exact opposite by buying the Vanguard S&P 500 ETF today. But that's not really the case if you're a long-term investor. In fact, history tells us that it's rarely a bad time to invest.

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Source Fool.com