This is a shocking deficiency with QuickBooks Online. I, too, somehow accidentally added a business as a Client instead of a Vendor. I caught my mistake during monthly reconciliation. Now I find out I cannot delete the errantly added Vendor, but only make them inactive. Alternatively, I can hand my credit card over to a third party company that implements such a basic feature through the QuickBooks API.

I tried submitting Feedback, but the Feedback button is broken.