JAS

Core Building Blocks: Large, Mid & Small Cap US ETFs

It seems to me that Vanguard's VV (Large Cap) effectively includes VO (Mid cap) -- reading their description, the VV tracks MSCI Prime 750, which the VO tracks MSCI Midcap 450, and the MSCI site says that the Prime 750 includes the Midcap 450! Does anyone have any idea why they "double-up" the midcap? I.E. My initial plan was to split 33% of my US allocation to each of VV/VO/VB, but on further examination I see that this would give me overexposure to the midcaps. Thoughts/comments? Thanks!