It probably is
The Select Vessel prompt will be the first item in the list so we can read that instead of the field name. The only time you might have a problem is if there are two similar dropdowns in the same document. Only one of them can be called 'drpVessel'. The macro therefore only processes the first such field. For multiple such fields in the same document, you would have to introduce a counter to add to the name.
Code:
Function Ddown(oDoc As Document) As Boolean
Dim oFF As FormField
Dim i As Long
On Error GoTo Err_Handler
For Each oFF In oDoc.FormFields
If oFF.Type = wdFieldFormDropDown Then
If InStr(1, LCase(oFF.Dropdown.ListEntries(1).Name), "select vessel") > 0 Then
With oFF
.Name = "drpVessel"
'With .Dropdown.ListEntries
' .Clear
' .Add "Item 1"
' .Add "Item 2"
' .Add "Item 3"
' .Add "Item 4"
'End With
End With
Exit For
End If
End If
Next oFF
Ddown = True
lbl_Exit:
Exit Function
Err_Handler:
Ddown = False
Resume lbl_Exit
End Function