Just mount the original image you is working with winreducer. This is necessary only for Professional x86 (I don't have tested other x86 images), just to copy the necessary dll files, and have sysprep working in VM enviroment. Once in VM enviroment, I have to delete the registry keys regarding SideShow.
Like this way:
1. Mont the image, and copy it to another place. Don't unmount yet.
2. Apply all updates you want in the mounted image, as explained before to save some time, and unmount it.
3. Do all modifications you want with winreducer.
4. Install windows with result ISO with a VM, and enter sysprep. Do all modifications you want, and finish sysprep.
5. If sysprep stops, just copy the files it needs from the copied original image in step 1 to VM enviroment, and finish it again.
6. In Disk Management in host, attach the VHD and capture it to a new WIM file. Replace the WIM generated in step 3 with this one.
For x64 Professional (again, I don't have tested other images), don't need to copy files or delete registry keys.