The long-term data for stock market returns paints a clear picture.The Vanguard Total Stock Market Index Fund ETF (VTI 0.27%) is one of the most popular exchange-traded funds (ETFs) on the planet. The fund has net assets of nearly $2 trillion. With stock indexes hovering near all-time highs, many investors are worried that this historically successful ETF will struggle in the years to come. But there's one critical piece of data that suggests otherwise.This ETF remains a data-backed investmentThe Vanguard T ...