Hi!
It's more than just the boot.wim file...
It might be for example that there is fragmentation on your disk which interferes.
Try to do the same VBOOT with an OSD script. All the VBOOT commands are in the first part of the script, before BEGINWINPE=TRUE, so you can schedule the script on your working Win7 device and you will have a step-by-step result of the VBOOT preparation in the logfiles.
Frank
Axle-IT