Technically, Schwab US Broad Market (SCHB) lacks about ~1000 stocks that are found in VTSAX / VTI. So in terms of not being "substantially identical", the points in favor are different brand names (Schwab vs Vanguard), different assets (~2500 vs ~3700 stocks), and different indices (DJ vs CSRP). You could also use the Russell 1000 (the 1000 largest stocks).