One of the more important parts of smart investing is diversification. This has two benefits: It reduces risks by not relying on too few stocks, and it increases your long-term return potential.

At The Motley Fool, we recommend investors have at least 25 stocks in their portfolio. Luckily, this doesn't have to involve investing in 25-plus individual stocks. It can be accomplished using exchange-traded funds (ETFs). ETFs allow you to invest in multiple companies at once and are a great way to achieve diversification without taking on the risks that come with individual stocks.

If you're looking to develop a well-rounded stock portfolio, the following four Vanguard ETFs can be your ticket. They check many boxes that investors should look for when investing long-term.

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1. Vanguard S&P 500 ETF

The Vanguard S&P 500 ETF (VOO -0.24%) is the foundation of my stock portfolio, and that likely won't change. It mirrors the S&P 500, which tracks the 500 largest U.S. companies on the market.

I like to call investing in this ETF an investment in the broader U.S. economy. Granted, it only contains large-cap stocks, so it doesn't fully represent the U.S. economy, but the companies it does contain contribute a significant amount to the economy and its growth.

The S&P 500 has become more tech-heavy than usual because of growing big tech valuations (it's market-cap-weighted), but it still contains companies from all 11 major sectors. Here's how they're represented (as of Feb. 28):

• Information Technology: 30.7%
• Financials: 14.5%
• Health Care: 10.8%
• Consumer Discretionary: 10.5%
• Communication Services: 9.4%
• Industrials: 8.3%
• Consumer Staples: 5.9%
• Energy: 3.3%
• Utilities: 2.4%
• Real Estate: 2.2%
• Materials: 2%

Since this ETF was created in September 2010, it has averaged close to 12% annual returns, which is impressive for a broad ETF. Past results don't guarantee future performance, but it can be a great long-term investment even if its annual average hovers around 10% (the historical S&P 500 average).

VOO data by YCharts.

2. The Vanguard Small-Cap ETF

The Vanguard Small-Cap ETF (VB -0.65%) is on the opposite end of the spectrum, containing only small-cap companies (those with a market capitalization between $300 million and $2 billion).

This ETF doesn't follow the Russell 2000 index like many other small-cap ETFs, but it has outperformed the index since it was created in January 2004.

VB data by YCharts.

Small-cap stocks are generally higher-risk, higher-reward investments. They're usually more volatile because they're more sensitive to the economic conditions. However, the smaller size also gives them more growth potential.

This doesn't mean that small-cap companies are early stage companies. Plenty of well-established businesses operating in niche markets are small-cap companies.

You probably wouldn't want a large percentage of your portfolio in small-cap stocks, but they can be a great complement, especially during economic expansions when they've often outperformed the broader market.

3. The Vanguard Mid-Cap ETF

The Vanguard Mid-Cap ETF (VO -0.34%) is the sweet spot between large-cap and small-cap stocks. Mid-cap stocks are large enough to have typically met their market fit and have a sustainable business model, but small enough to still have high-growth opportunities in front of them.

Having stocks with a good mix of stability and growth is good for balancing risk and reward. This ETF is smaller than the large-cap and small-cap options at only 318 stocks, but it covers a lot of ground sector-wise:

• Industrials: 19.4%
• Technology: 14%
• Financials: 13.9%
• Consumer Discretionary: 12.7%
• Utilities: 8.5%
• Health Care: 8.4%
• Real Estate: 7.9%
• Consumer Staples: 6%
• Energy: 5.6%
• Basic Materials: 2.7%
• Telecommunications: 0.9%

Every investor is different, but I'd set the max mid-cap representation in my stock portfolio at around 10%.

4. Vanguard Total International Stock ETF

Your portfolio isn't truly diversified if it only contains U.S. companies. It's nice to have international stocks because they aren't as tied to the U.S. economy and present different opportunities.

One of the best ways to get exposure to international stocks is through the Vanguard Total International Stock ETF (VXUS -0.78%), which contains over 1,900 companies from both developed and emerging markets. Investing in companies from both markets is beneficial because they have different risks and benefits.

Developed markets are typically less risky because they have more economic stability, but the opportunity for rapid growth may be limited. Emerging markets carry more risk because of increased volatility, potential political instability, and developing infrastructure, but high-growth chances are more readily available.

VXUS Dividend Yield data by YCharts.

Aside from geographic diversification, this ETF offers a dividend yield that is routinely double that of the S&P 500 average. Stock price appreciation is great, but having above-average dividend payouts can be just as rewarding in many cases. If you're investing for the long term, this ETF can be a great portfolio addition.