Currently Windows Vista Service Pack 2 is available for download on Microsoft Download Center. You can download Vista SP2 as a separate installer or an ISO image to burn on DVD.

The installer for Vista SP2 is about 350 MB but if you do not want to download that large file, stay until Microsoft move forwards to SP2 patch by Windows Update or Automatic Updates feature as that will only download important components.

The main point you will be able to install Windows Vista SP2 when machine is already running Service Pack 1. Microsoft states that they choose not to suggest a cumulative SP2 package this time to keep file size controlled.

If you have utilized vLite to slipstream Vista with SP1, you get certain errors while installing SP2 due to lost components. In that situation, the best choice is to either do an upgrade using original Windows Vista DVD or again install Vista from scratch, then install SP1 followed by a file clean-up and then update to Service Pack 2.