Set the memo field in the item line of the Purchase order using workflow.

Create a custom field in the vendor record and assign this value to the memo field of item line of the purchase order using workflow.

First create sublist action group for the items. Mention the trigger point of the workflow and sublist in the workflow.

Add set field value in the sublist action group in the workflow.

Specify the field, we need to assign the value in the purchase record and also mention the field in the vendor record for read the values .