RE: vendor item number on purchase order
I Agree with Aleksandar, You can use Cross reference functionality where you can set the vendor's Item code against your internal item code and this cross reference number is also available in PO line.
Second option, If Vendor's Item code is common across all vendors for an item then you can define it on Item Card.
Now, to print, Vendor's Item code instead of internal item code. you can make a condition in report that if Cross reference number is exist on PO line then it should print Cross reference number else Item code. This is very small modification that you need to do on purchase order report.
Same is applicable for second option of item card but in that case, you need to check item table for the vendor's Item code field value.
Saurabh shah