How can I add a Vendor as an employee? I paid this person as Vendor before hiring them, and now QB Payroll keeps saying the name is already in use.

Change the vendor name so it is slightly different.  The built in QB feature to do this is to use curly braces after the name, like {Vendor} in this case.

Use the Employee's legal name on the employee record, from their SS card.

QuickBooks puts up a message when you hit this case telling you what to do and about this feature. Following the advise on the message will get you to the solution.