because some fixes apply to WinPE, which is not being updates when you use vLite, MS doesn't offer the slipstreaming option inside the Sp1. vLite uses an unofficial hack to integrate the Sp1, so don't hassle MS about this. I'm sure you can find a Sp1 ISO somewhere inside the www To update the boot.wim, mount it with imagex, run the pkgmgr.exe to integrate the hotfix into the mounted folder and commit and unmount the WIM again. For more details about this see the WAIK documentation.