You're 20. Now is a PERFECT time to be 100% stocks. Just think of the money as a one-way street. Money only goes into that account. You can revisit that decision in 10-15 years.
So the key is that for each dollar you put in, think, do I really need that money? Will my life be horrible if I don't have it. If the answers are no and no, put it in the market, and thank yourself 15 years in the future.
As others said, the rate of return is the increase in value in your portfolio, which typically includes dividends (the default on Vanguard is to reinvest dividends by buying additional funds--at least for the index fund, not sure about ETF). But as mentioned above, it will be very volatile along the way. Some years up--and maybe way up, some years down--and maybe way down. But on average over many years, stocks tend to return an average of 7% per year, which is very good over the long-term.
So put money there if you don't need it, and be happy living on less. You will be very, very, very happy with that decision when you're older if you've stuck to it.