By system default, PO number is not directly reflected on VB.
If you maintain 1-1 relationship between VB and PO, then you can have a custom field and source the value using formulae.
If in future you plan to have 1 VB to many PO, you will have to deploy a script to identify the POs and tag it on custom field on VB.
Currently, the ability to show Purchase Order No. on the Vendor Bill aside from the Related Records tab is a system limitation.
Here are some related enhancements that I found:
- Enhancement 64905 Transactions > Bill Purchase Orders > Give users ability to have PO number display on the vendor bill.
- Enhancement 173052 Customize the Vendor Bill Transaction Form > Ability to display the Created From Field showing the Purchase Order Number.