Thank you for elaborating on your feature request.
I agree that having an additional custom field for expenses would be very useful for tracking expenses. I have filed your suggestion to our Product team so they can have in mind this feature improvement for future development. For the Time being, I would suggest creating separate VAT categories for expenses.
I agree that having a VAT expense category without any reference to expense could be confusing rest assured that your feedback has been forwarded to the Product team so they can have it in mind for future development.