Investing a regular amount of money into the stock market each month can be an excellent way to grow your savings and build up a portfolio that's eventually worth $1 million or more. But it can be challenging to do, especially since you have to ensure you can continue to afford making monthly investments, and then picking which investments to make with that money. Amid volatile economic conditions, that's no easy task.

You can, however, simplify the process by going with some solid exchange-traded funds (ETFs) that can diversify your portfolio and set you up for some great growth opportunities in the future. A couple of low-cost Vanguard ETFs to consider for this purpose include the Vanguard Growth Index Fund (VUG -0.43%) and the Vanguard Information Technology Index Fund (VGT -0.36%).

Here's why investing $300 per month into either one of these ETFs could put you on track to generating a $1 million portfolio in the future.

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Vanguard Growth Index Fund

The Vanguard Growth Index Fund is a great, growth-focused ETF you can add to your portfolio. It charges an expense ratio of only 0.04%, which means you don't have to worry about high fees chipping away at your gains.

What's attractive about this fund is that it focuses on large-cap growth stocks. These are the types of investments that can drive long-run returns for your portfolio and make the most of your money. Stocks such as Tesla, Amazon, and Nvidia are all among its top-10 holdings. These are leaders within their respective industries, and their businesses are synonymous with growth. With more than 160 stocks in total, this is a well-diversified ETF to simply buy and hold. It also yields around 0.5%.

Over the past decade, the ETF has achieved total returns (which include dividend payments) of approximately 327%. That averages out to a compound annual growth rate (CAGR) of 15.6%.

But for the sake of being conservative, let's assume that its returns will slow down given how hot the market has been in the past few years and how it's reaching record levels. If the ETF averages a return of about 10% for the very long haul (which is in line with the S&P 500's long-term average), then a $300 per-month investment could grow to more than $1 million after a period of 34 years.

This would require investing in the ETF every month during that time frame. But by doing so, you can put yourself on a path to producing some fantastic returns thanks to the effects of compounding.

VUG Total Return Level Chart

VUG Total Return Level data by YCharts.

Vanguard Information Technology Index Fund

As terrific of a growth investment as the Vanguard Growth Index Fund has been in recent years, it still falls well short of the gains the Vanguard Information Technology Index Fund has produced during that stretch. At 543%, its 10-year total returns average out to an annual gain of 20.5%.

That's a mind-boggling return, and it highlights just how impressive the stocks within this ETF have been. There will be some overlap between this fund and the growth ETF, but the big difference is there is heavier exposure to big tech. Nvidia, Microsoft, and Apple account for a combined 45% of the Vanguard Information Technology ETF's total holdings, but they make up just around 32% of the growth ETF. That difference can be substantial over time, especially given how well a massive stock like Nvidia has performed. In 10 years, its returns have been truly exceptional, totaling 28,000%.

Given Nvidia's size today as one of the most valuable companies in the world, odds are its returns will be far more modest over the next decade. While they may still be great, it's probably a good idea to factor in a healthy dose of conservatism with this ETF as well given how much of a boost Nvidia has given it in the past. Even though the ETF is focused on tech and growth, averaging 20% annual returns likely isn't going to be sustainable over the very long haul. The expectation of a 10% return may also be prudent with this ETF to ensure your expectations aren't set too high for future gains.

As with the growth ETF, if you invest $300 per month into this fund, you can also be on the path to a $1 million portfolio. If this ETF continues to outperform the market, however, then it may take less than 34 years to get to $1 million. But by staying the course and investing regularly into this or the growth ETF, you can be in a good position for building up a solid portfolio over the long haul.

The Vanguard Information Technology ETF charges an expense ratio of 0.09%, and while that's a bit higher than the growth ETF's fees, they aren't going to drastically alter your prospects for generating potentially life-changing returns from regularly investing in this fund.