No. The DirectX installer is pretty smart, it will NOT overwrite your directX 10 files that are on vista, it may even go so far as to say that the install is not necessary due to it being an older DX (and therefore exit the install); however, if the case is that your DX10 package is missing some DX6 files, those should get added in.
I had to do something similar a few years ago, somehow my DX9 install was completely missing a bunch of DX8 stuff, to the point where D3D7 and D3D9 would work but D3D8 would fail spectacularly. Running the DX8.1 redistrubutable added in the missing files, despite the fact it saw that my current version of DX was 9.0