I appreciate you taking the time to post your question here in the Community. I can help you with converting a vendor to an employee in QuickBooks Online.
In order to convert a vendor to an employee, you'll need to make the employee with a slightly different name (maybe add the middle initial). You can either use Intuit Smith as the vendor and then Intuit L. Smith as the employee, This is to make sure that the one is never controlling the other.
Once you're done, you have the option to make the vendor inactive.
I hope this response finds you with a smile. Let me know if I can be of additional assistance. Cheering you and your business to continued prosperity.