Hi @JohnDaker, welcome to the Obsidian community!
To share another point of view, I use many different vaults and it works very well for me.
At first, I tried to keep everything in just two vaults - one for work and one for my personal life. But I found that the vaults quickly became overwhelming with their mix of reference, projects, creative work, and many other things. I’m pretty distractible, and I need my vaults to be well organized, so I realized I needed hard boundaries to filter out information that’s not related to the task at hand.
Now I prefer to have each vault be focused on a particular sphere of work, and not be distracted by notes that don’t relate to the effort at hand. If I were starting my master’s thesis, I would create a new vault for it. (Since a vault is just a folder of files, I can always merge the vault with another one later if I want.)
Here are a few examples of the kind of information I keep in separate vaults:
Work Vault: This is a vault where I track my projects, products, contacts, and reference for work. Anything specific to my employer goes here. It lives on my work laptop, is backed up to my work network, and is never synced anywhere. I keep this in a separate vault so that it can never be accidentally mixed with other work.
Reference Vault: This is a vault where I keep my reference notes that are not specific to my employer. I’m a software developer, so this is where I keep general knowledge about languages, formats, algorithms, and many other things. This vault is useful to me both professionally and personally, so I sync it across most of my devices.
Project Vaults: Outside of work I am engaged in lots of creative projects, such as table-top RPGs. Each story’s world gets its own vault, where I track people, places, plot, and more. I rely heavily on Dataview to provide me with reports, so keeping each vault focused on its own world keeps organization and queries as simple as possible.
I hope this helps – if nothing else I hope it highlights that different approaches work for different people. I suggest you organize your vault(s) however seems best at first, and be ready to re-organize as you discover the true shape of your data. 
Craig