The supported method for creating answer files for unattended installation of Windows Vista is to use the Windows System Image Manager (Windows SIM). You could also create answer files manually using Notepad if you wanted to, but you’d better validate your answer files using Windows SIM since XML syntax can be messy to work with by hand.
Here’s a third alternative I’ve found: this web page lets you create a Vista answer file by making your selections using a forms-based interface. Two caveats: there are only a limited number of settings you can configure, and I can’t vouch for the group who maintains this page as I don’t know them, so this recommendation is presented to you as-is with no guarantees!
For more information on deploying Vista, see my series of articles titled Deploying Vista right here on WindowsNetworking.com.