By now most Vista users would have already updated to Windows Vista SP2. For those who wish to download and save its stand alone installer, you can also download it from the Microsoft Download Center.
Service Pack 2 for Windows Server 2008 and Windows Vista (SP2) is an update to Windows Vista and Windows Server 2008 that supports new kinds of hardware and emerging hardware standards, and includes all updates delivered since SP1. SP2 simplifies administration by enabling IT administrators to deploy and support a single service pack for clients and servers.
SP2 is an update to Windows Server 2008 and Windows Vista that incorporates improvements discovered through automated feedback, as well as updates that have been delivered since SP1. By providing these fixes integrated into a single service pack for both client and server, Microsoft provides a single high-quality update that minimizes deployment and testing complexity for customers.
Service Pack 1 is a prerequisite for installing Service Pack 2. Please make sure that your system is running Service Pack 1 before you install Service Pack 2.
Windows Vista Service Pack 2 Download Link
Microsoft Download Center : Service Pack 2 for Windows Vista.
For more see KB948465.