Okay. I'm pretty sure that vector files are the actual audio data, and program files are the indexing tool. The table of contents for Nebula to work with. There are tons of parameters to play with, so when you edit any volume or kernel stuff or anything within Nebula, and you hit save, you're modifying the program file.
The program files released by Francesco are supposed to go with the vector files you already have downloaded from me or Marcus.
The differences really aren't all that enormous, but do make a very small difference. I'm beginning to think the problem is with the sound of Nebula, or something to do with the NAT sampling process.