No it doesn't come installed on vista, you have to download it. I know of only one game (demo, really) that uses DX10... "Lost Planet: Extreme Conditions". I'll note that it doesn't run on Vista with a GeForce 7900 GTX, but that's not a DX-10 card anyways (and I only have DX-9c installed).
__________________
Primary
CPU: Athlon 64 X2 4400+ Mobo: Biostar N4SLI-A9 RAM: 2G Crucial (DDR400) Video: eVGA GeForce 7900 GTX (512M) Audio: HDA X-Mystique HD(s): Maxtor 300G SATA2, Samsung 400G SATA2 OS(s): WinXP x64 Pro, Vista x32 Ultimate, Gentoo x64 Monitor(s): Primary - 19" Flat Panel (1280x1024) Secondary - 19" Flat Panel (1280x1024) Tertiary - Zenith 42" Plasma TV (1024x768 res)
Many other machines... sig too short