VBoot Loader loads itself to the real mode memory address 0x90000 (576k), and it occupies 64k, i.e., 576K-640K is used by VBoot, this memory region is most likely free to use. However, if you are getting this error, it indicates there is something else already using this region. Make sure you don't have other boot loaders, don't chainload to VBoot from other loaders, don't use VBoot on encrypted hard disks, etc.