Function for searching via vlookup in closed workbook is not working

Question

I try to do a Vlookup in a closed workbook via ExecuteExcel4Macro. Since I need to find a lot of values in different workbooks it makes sense to do it in a closed workbook.

I used the below code, but it doesn't work if I enter the function on my Excel book:

=GetVlookup("D:", "testfile.xls", "Sheet1", "C1:D8", 2, "CDM-01_10")

I can assure that there is a file on my D: named testfile.xls with a sheet1 and content in the range I mention here. But result is #value!.

I want this in a function since I want to make a price calculation add-in that can be used in several workbooks, so I rather haven't introduced this in a macro.

Public Function GetVlookup(path, file, sheet, ref, Col, vVal)
'   Retrieves a value from a closed workbook

    Dim arg As String

'   Make sure the file exists

If Right(path, 1) <> "\" Then path = path & "\"

If Dir(path & file) = "" Then
    GetVlookup = "File Not Found"
    Exit Function
End If

If IsNumeric(vVal) Then
  vVal = CDbl(vVal)
Else
  vVal = Chr(34) & vVal & Chr(34)
  End If

' Create the argument
arg = "'" & path & "[" & file & "]" & sheet & "'!" & Range(ref).Address(, , xlR1C1)
' Execute an XLM macro
GetVlookup = ExecuteExcel4Macro("Vlookup(" & vVal & "," & arg & "," & Col & ",0)")
End Function