(Note: All of the information here is accurate as of Vista build 5536 [Pre-RC1]. This information may change in later builds.)
(Update: For updated information, go to VistaGameDoctor.com. You'll need three seperate articles: Steam, SiN (Steam release) and SiN Episodes: Emergence.)
Here is the compatibility information that you are going to need to get "SiN" and "SiN Episodes: Emergence" working in Vista.
Steam: In a default configuration, Steam does not work on Windows Vista. In order to get Steam to work in Vista, you must right-click on your desktop shortcut for Steam and select "Run as administrator," even if you are running in an administrator-level account. If you do not, Steam will either error out after the next Steam update, or will fail to update your GCF's when a patch is released.
If you do elevate it to administrator privledges, you will not be able to close Steam by right-clicking on the tray icon and picking "Exit." After doing that, you will have to open Task Manager, list all tasks for all processes, and end-task Steam.exe manually.
Clarification: Steam will work fine until either the Steam client is updated, or you restart Steam to get an update to a game. At that point, you'll encounter issues due to the way that Vista handles file system security for the Program Files folder.
SiN Episodes: Once Steam is working in Vista, "SiN Episodes: Emergence" works fine using the built-in drivers for nVidia and ATI. In other words, as long as you have a DX9.0c-level card, you should be fine.
SiN: At the moment, "SiN" will not work in Vista without a driver update. Vista only ships with installed support for OpenGL v1.1 (driver: Microsoft Corporation GDI Generic version 1.1.0). You will have to update to the latest Vista drivers for your video card in order to play "SiN" either from the original disc or from Steam.