Many people think that to end up a millionaire, they need to build a complex portfolio full of carefully selected stocks. And while that may, in fact, be one ticket to millionaire status, it's also possible to grow a tremendous amount of wealth without doing a lot of work at all. If that sounds like a good option to you, then it pays to look at the Vanguard S&P 500 ETF (NYSEMKT: VOO). The upside of investing in the S&P 500 The S&P 500 index consists of the 500 largest publicly traded companies, so it's a great representation of the broad market. When you invest in the S&P 500, you don't just add quality stocks to your portfolio -- you also get instant diversification. Image source: Getty Images. The Vanguard S&P 500 ETF tracks the S&P 500. (If you're not familiar with ETFs, that's short for exchange-traded funds. These funds follow different market indexes and let you scoop up a bucket of stocks with a single investment. It really doesn't get more convenient than that.) Now the bad news is that the fund's goal isn't to beat the S&P 500. The good news, however, is that the S&P 500 has had a strong performance over time, and so matching it is a good benchmark to aim for. In fact, the Vanguard S&P 500 ETF has delivered an average annual 15% return since its inception. How a single ETF could make you a millionaire The Vanguard S&P 500 ETF is a great choice as an investment, but let's be clear -- you'll need to stick with it for a long time for it to turn you into a millionaire. But let's assume you put $10,000 into that ETF and leave your money alone for 35 years. If that 15% average annual return holds, you'll wind up with $1.33 million. Keep your $10,000 invested for 40 years, and you'll be looking at almost $2.7 million. Of course, not everyone has $10,000 on hand to sink into a single ETF. So let's say that instead of a lump sum investment, you buy shares over time. If you invest $100 a month over 35 years, you'll wind up with just over $1 million -- again, assuming the fund delivers that same average annual 15% return during that window. An easy ticket to wealth The Vanguard S&P 500 ETF isn't the only S&P 500 ETF you can scoop up. But it's a good choice if you want to accrue a lot of money without having to put a lot of thought or effort into it. Of course, hand-picking a select group of S&P 500 stocks could give you a significantly higher return over time than what the Vanguard S&P 500 ETF will give you. But if you're happy with the idea of a 15% yearly return and you're not looking to spend a lot of time researching companies and updating your portfolio, then this is one investment that could end up serving you very well for many years to come. 10 stocks we like better than Vanguard S&P 500 ETFWhen investing geniuses David and Tom Gardner have a stock tip, it can pay to listen. After all, the newsletter they have run for over a decade, Motley Fool Stock Advisor, has tripled the market.* David and Tom just revealed what they believe are the ten best stocks for investors to buy right now... and Vanguard S&P 500 ETF wasn't one of them! That's right -- they think these 10 stocks are even better buys. See the 10 stocks *Stock Advisor returns as of February 24, 2021 Maurie Backman has no position in any of the stocks mentioned. The Motley Fool owns shares of Vanguard S&P 500 ETF. The Motley Fool has a disclosure policy.Source