As Windows 7, codenamed Vienna is an improved version of Vista, DirectX 11 will work on both systems. It looks that DirectX 11 will launch before Windows 7 and we have high hopes of seeing DirectX 11 by the end of 2009.
DX11 is based on DirectX 10 but naturally it has more options including Tessellation, compute Shaders - something good for Cuda and GPGPU, CPU multithread support and some new texture compression. Lets not forget Shader model 5.0 but at this time we dont know enough about it.
Tessellation might be the most important feature as some developers are getting excited about it. Tessellation can subdivide objects and give them better level of details and more polygons but we dont know what implementation Microsoft plans for DirectX 11.
The important thing is that despite being designed for Windows 7, DirectX 11 will certainly launch before the new OS and at the same time it should run well on Vista.