I’ve had the same struggles to decide if I should use one or several vaults.
I came up with the following solution:
As we can see each vault being a separate folder, there is the main folder that contains all the vaults, right? Well, I just open that main folder as a vault in Obsidian and it will contain all other vaults within and also shows me the linked information. If I need to zoom in and focus on a specific area, I could open that subfolder or “sub-vault” for that matter.
Does this make sense or am I missing anything?