I go against recommended practice and have different vaults for different things in my life. The academic note vault is separate from the personal vault is separate from the creative projects vault. I have also committed sacrilege by not having many notes linked to each other. I’m trying to migrate a lot of notes from Google Docs and Notion over into Obsidian, so all of the vaults are pretty messy.
I love the LaTeX integration. Lots of math formulas in the academic note vault. I use the callout feature everywhere. I also nest callouts in callouts. I’m frankly treating them as equivalent to toggles in Notion.
I most often go to the personal vault where I have a list of things I’ve 1) seen online before, 2) spent at least an hour trying to refind that thing later and 3) will probably want to find again. This way I don’t lose time trying to find it again. It’s really helpful for me. I also have a list of food brands and how much I liked them, so I can remember which brand of turkey was bad and which was tolerable and which I’d definitely buy again.
To me the only reason for a second vault is for sharing or collaborating with others. Obsidian really doesn't care how many notes you have in your vault so maintaining separate vaults just adds additional steps in getting to your info. (Obsidian doesn't care about directories either for that matter.)