Importing of Customer and Vendor Contacts Via DMF data entity is lacking in its structure.
There can be 1:N Customers OR Vendors to a ContactPerson
ContactPerson table has CustAccount, but it is never used in the smmContactPersonV2Entity [ContactsV2] Data Entity for loading of Contacts. Why isnt VendAccount on the table?
You can even split them into distinct use DE's like was done for CustomerVendorExternalItem Entity same exact motif holds for Module Enum used.
Suggestion is to use standard MSFT functionality of Account Relation and Relation type Module :: Cust | Vend | etc.
The Resultant Data entity changes allow the DMF user to have Customer or Vendor Account on the Excel Spreadsheet to lookup the necessary ContactForParty RefRecId rather than a CalculatedField method to call SQL directly.
XPP Code to retrieve Party [Recid] from Cust or Vend Table given AccountNum
ContactPerson.ContactForParty = Cust or VendTable.Party