Let me share some insights about managing user access in QuickBooks Online (QBO).
When adding a user in your company file, you have an option to select access rights for limited information from the vendor where the user can create Purchase Order (PO) or any other related to vendor's transactions.
However, the user can still be able to view all expenses and there isn't a way to hide it.
I can see how this option would be beneficial for you. I'll make sure to take note of this and will inform you once we receive product enhancement update about the availability of this option. Please know that the change is also based on how popular a specific feature request is.
In the meantime, I'd suggest visiting our QuickBooks Online Blog regularly. This way you'll be updated with the latest news and updates including product improvements.