The search of the Associative Search Database returns its results as a CscXDocField object. Search for a string can be done as follows:

Private Function QueryVendorOnInvoiceVendors(VendorName As String) As CscXDocField

   Dim vendorDatabase As CscAssociativeSearchDatabase
   Set vendorDatabase = Project.Databases.ItemByName("InvoiceVendors").AssociativeSearch

   Set QueryVendorOnInvoiceVendors = vendorDatabase.SearchText(VendorName)
End Function

Searching for a complete document can be done as follows:

Private Function QueryDocumentOnInvoiceVendors(ByVal pXDoc As CASCADELib.CscXDocument) As CscXDocField

   Dim vendorDatabase As CscAssociativeSearchDatabase
   Set vendorDatabase = Project.Databases.ItemByName("InvoiceVendors").AssociativeSearch

   Set QueryDocumentOnInvoiceVendors = vendorDatabase.SearchDocument(pXDoc)
End Function