Vendor Memos may be entered in "Vendor Memo Entry" (3.5.1.4.1). There are two types: a Purchase Order Memo which may be used to let the purchasing agent know that you need to order a certain item. An Accounts Payable Memo that can be used as a reminder to discuss a particular issue with the Vendor's Receivable department.
You may generate a list of memos in "Vendor Memo Report" (3.5.1.4.2).