johannesleung,
I've been getting the exact same problem with the vibrant morrowind textures not loading after my cache is cleared. The problem may be that they are too large for the xbox to load. If you havent noticed, the interiors look especially crisper, not to mention many of the subtle details like building trims are more detailed in the virbant 2.0 textures. The only way i can guess to fix this problem would be to resize many of the vibrant textures to a smaller resolution. In perfect edition there are 4,000 some odd textures added and they all work fine, compared to vibrant's textures which don't and are just over 1,000. Most of the perfect edition replacement textures are also half the file size of vibrant's, which is why I think the size is the issue.
The x-box can handle loading them before it's cache is cleared, and I wish there were some otherway to fix this, because the vibrant texture conversion is so much crisper at its current resolutions.