I just circled back to this question and took a closer look at one of the existing Vector disease modules in S3PE. Looks like they are all tuning to me, while it's the base mod that is the script code. Seems to me that one could take a copy of an existing module and change it up to add some new diseases. There may be more to it than that, but if one is familiar with S3PE and XML code editing such as through creating and editing one's own tuning mods, this should be pretty much within reach for anyone who wishes to try.
Would still recommend bringing the question to our developer, though. The difficult part may be getting the Vector base mod to properly recognize the altered creation as valid diseases distinct from the originals to manage.