First of all, NO windows Vista won't include DX10.
Second, the 7800 series are 100% windows Vista certified, so you won't have any troubles running it on a PC with these cards.
Third, windows Vista will integrate the videocard as a more integral part of the PC, they'll even share recourses with other components.
Lastly I want to say that DX10 is a major threat to the console industry. The advent of Geometry Shaders will unleash a whole new world of possibilities for game programmers.