ENTER VENDOR CODE NUMBERS, i.e. ABC01 for the ABC Machine Co.
ENTERING VENDOR CODE NUMBERS HOW DO I ENTER VENDOR CODE NUMBERS
ENTERING VENDOR CODE NUMBERS HOW DO I ENTER VENDOR CODE NUMBERS

If you are asking where you can enter your own vendor code to pull up the vendor from the drop down menu, you would put the code in the Vendor Name Box. This is how it will show up to you. Then in the Company Name box, you would put the actual name of the business. Make sure that you double check the Print on Check as box to make sure the correct name is there.
This is all done under the Vendor Center, right clicking on the vendor and choosing edit.