To update the WIM files, you need to install the WIM file on a computer, then install SP1. Next, run SYSPREP on the machine and recapture the image to a new WIM file. Normally, running this SYSPREP would decrement the Vista installation re-arm count. However, because the SP1 installation requires an image recapture, Microsoft included an extra re-arm.
Make sure you use the latest Windows Automated Installation Kit (WAIK) version at Automated Installation Kit (AIK) for Windows Vista SP1 and Windows Server 2008 . It supports Windows Server 2008 and Vista SP1.