When booting from an USB stick it is important how you are formating the stick. XPe / WES comes with the UFDPrep.exe utility that helps You to prepare the flash device,
but it runs on XP only.
When formatted with Vista, USB sticks are bootable, but only for EFI boot.
Therefore they are booting e.g. WinPE 2.0 (based on the VISTA kernel) perfectly, but with XPe /WES You will noz be sucessfull.
If Your development system is Vista, You might apply a little trick: Use the first disk of XPe / WES to boot into WinPE 1.5 (which is based on the XP kernel) and run UFDPrep. Copy over the image and all should be fine. If You have included the USB boot components, of course.