Can I reclassify a vendor to Employee and still have the transactions that were posted under vendor follow ?

I'll help you make sure that your payroll records are correct, pampcp.

For now, we're unable to move or convert vendor transactions to the Employee Center.

If you've already added the vendor as an employee, you can create a year-to-date adjustment so you can lump sum the amounts when recording the data. Here's how:

1. Go to the Help menu and select About QuickBooks.
2. Press CTRL + ALT + Y simultaneously.
3. Click Next thrice until you reached the Employee summary information window.
4. Highlight the name of the employee and click Enter Summary.
5. Make sure that the From and To dates are correct.
6. Enter all other details and amounts.
7. Click OK.

Here's an article for more details: Enter Historical Payroll Data.

Let me know if you need additional help when recording the adjustment.