I’ve a single vault with everything in it - personal journal, research projects, writing projects, random notes, all of it. I separate it all in a folder structure when I can, and use tags for finding the random notes and files otherwise. I’ve got notes from Kindle and Zotero books that get pulled in when necessary, as well as Fleeting Notes. I keep my vault synced on my desktop, laptop, and Android phone via a home-grown GitHub sync script.
I have a large collection of stuff in a OneNote notebook, which stays there until I need it. No sense taking the time to move over stuff I may not need right now.
I do a lot of my writing in it using Longform, as well as just in a note for shorter pieces. Those include some crappy creative writing and slightly less crappy technical articles.