As stated before, The Vault tries not to split files between disks but it will do so if necessary to avoid leaving a large unfilled ``hole" in a disk. In more detail, The Vault will first fill up a disk as well as it can without splitting files. If the disk isn't full, it then looks for files which are larger than a preset ``split threshold". If there are any, it will fill up the disk by splitting a file; if not, it will go on to the next disk. Thus, the split threshold is both the smallest size file that will ever be split and the largest amount of empty space that The Vault will ever leave on a disk (except for the last).
You can set the split threshold by selecting `Set Split Threshold' under the `Options' menu. The threshold is normally set at 300,000 bytes. The split threshold should always be smaller than the smallest disk you will be using; otherwise, The Vault will not work correctly if it comes to a file which is larger than the disk.