For most of us, just getting by these days is tough enough, having to pay off a mortgage, cars, kids' educations, and everything else while trying to stay employed during the worst pandemic in 100 years. The thought of being a millionaire is the furthest thing from many of our minds. But with smart, disciplined investing, that type of financial security could be achieved in retirement.

You don't necessarily need a huge portfolio or a broker to advise you. You can achieve the type of diversification and return you need in a single investment -- an exchange-traded fund (ETF), the Vanguard Extended Market Index ETF (VXF -0.10%), in particular. Investing now in this all-cap fund, combined with your 401(k) and Social Security, could make you a millionaire in retirement.

Smiling senior man standing next to  convertible

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A long-term winner

There are a lot of great ETFs available to investors among the thousands on the market. But there are a few reasons why the Vanguard Extended Market Index ETF stands out as a good choice for long-term investors. If you are the type of investor who wants just one extra fund -- in addition to your employer-sponsored plan or individual retirement account -- to set and forget, this is a good choice.

It holds basically the entire spectrum of the stock market outside of the S&P 500, tracking the S&P Completion Index. The S&P Completion Index includes over 3,000 stocks, offering exposure to all the mid-, small-, and micro-cap stocks outside the S&P 500.

The Vanguard Extended Market Index ETF actually holds about 3,371 stocks, with a median market cap of $7.1 billion. Its largest holdings are Square, Zoom, and Uber, with the largest holding at just over 1% of the portfolio.

Small- and mid-cap stocks are typically more volatile in the short term than large-caps, but over time, the small- and mid-caps have generally outperformed large-caps. Small- and mid-caps typically perform better in times of economic turbulence, while large-caps lead the way during bull markets.

Since inception on Dec. 27, 2001, the Vanguard Extended Market ETF has returned 11% through Feb. 28, 2021 -- that's about 20 years, including a global recession, a bull market, and a pandemic. Over that same time period, the S&P 500 has returned roughly 8% on an annual basis -- that's significant outperformance. What's also great about this ETF is its low expense ratio -- a minuscule 0.06%, well below the category average.

A millionaire-maker

While past performance is no guarantee of future results, for argument's sake, lets say the Vanguard Extended Market ETF has the same type of performance over the next 20 years as it had over the last 20 years. In 20 years, a $30,000 investment in this ETF with an 11% annual return, contributing $200 per month, would amount to about $442,000. If you have a longer time horizon and held this ETF for 30 years, contributing $200 per month, with an 11% annual return, you would have more than $1.3 million.

There is, of course, no guarantee that this ETF will return 11% per year for the next 30 years, but even a 10% return would net you $1 million after 30 years. The bottom line is -- no matter where you are on the road to retirement, the longer you have to make your money work, the better off you will be in the long run. And this ETF, in particular, is built for long-term performance.

It is impossible to predict what the next 20 or 30 years will look like on the markets, but with this ETF, you know you have the diversification to weather the ups and downs, based on its strong performance over these turbulent last 20 years.