Are CBFS Storage vaults subject to fragmentation?

In short: No, fragmentation is not a concern for CBFS Storage vaults.

Fragmentation is a problem unique to physical HDD disks with moving heads: to read and write sectors, the HDD has to reposition heads and this slows down reading and writing. On modern multicore processors, where a number of files are typically being read and written in parallel, fragmentation is not as influential as it was in the MS-DOS days. Additionally, fragmentation is not applicable for flash-based storage (SSDs, USB drives, flash cards, etc).

In the case of CBFS Storage, usually a vault is stored as a file on the regular disk (an HDD, SSD, etc.), so it doesn't matter to CBFS Storage engine directly whether the file data within the vault is stored sequentially or not since the vault file may or may not be fragmented itself. If the vault is stored remotely or in memory, the sequence of the pages doesn't matter, either.