Just days after making this new version of Windows Vista Service Pack 1 (SP1) Release Candidate Refresh available to testers, Microsoft has posted the release publicly:

Windows Vista Service Pack 1 RC Refresh is available through Windows Update.

Install Windows Vista SP1 Release Candidate through Windows Update
Windows Vista Service Pack 1 RC Refresh is available to Beta testers through Windows Update. Experience the WU installation path for Windows Vista SP1 by following these simple steps and give us your feedback to improve our quality.

Systems running on Windows Vista RTM typically require two or three updates before SP1 can be installed. These updates are permanent on your Windows Vista systems. Windows Update will detect your system configuration and offer the prerequisite packages that are applicable to your system. Based on the state of your system, Windows Update will offer you all the applicable prerequisite packages and SP1 in a sequential order.

If you're already running the December version of SP1 RC, you should look into updating.