I keep nearly all of my notes in one Obsidian vault. I’ve found that cross-linking and my daily journaling have benefited the most from keeping everything in one directory and thus vault.
I do have separate vaults for dedicated topics of longer form writing. I have an ongoing blog series that has its own vault. And I have another for a topic that may turn into a book.
This series started as one note in the main vault.
I keep the all of my vaults as subdirectories in one directory that lives online for ease of access from different computers and mobile devices. At the moment this is on Dropbox.