mimagephotography and Nastassia Samal from Getty Images There really aren't two more vital and closely-watched exchange traded funds (ETFs) out there than the Vanguard S&P 500 ETF (VOO) and the Vanguard Total Stock Market Index Fund (VTI). Indeed, these are two top holdings of mine, and I do look at rebalancing them from time to time with my own set of concentration criteria. On this front, every individual investor will differ. That said, I think there are important differences between the two funds wor ...