1. VTMSX might be better in taxed accounts, while VSIAX might be better in sheltered accounts. S&P has gotten good small cap fund returns because they only accept companies with positive earnings. But small cap growth has done very well recently, which might make it better to look at VSIAX. Who knows, really.
2. Who knows, really. There is a theory that small cap value will do better, or at least be less correlated with the total market. If you don't want to test this then maybe you should also avoid overweighting small caps.
3. Low expenses, persistent strategy, not likely to disappear.