Modest annual returns can create life-changing wealth over the long term, thanks to the magic of compounding.The Vanguard Total Stock Market ETF (VTI 0.38%) is one of the most diversified exchange-traded funds (ETFs) money can buy. It tracks the performance of the CRSP U.S. Total Market Index, which invests in all 3,508 companies listed on American stock exchanges, so it's a complete portfolio on its own.But there are pros and cons to extreme diversification. On the one hand, it can reduce volatility, which ...