Il codice corretto potrebbe essere questo

Ovviamente cambia il percorso e il nome dei file

codice:
        Dim VBScript As New MSScriptControl.ScriptControl
        Dim Code As String
        Dim Result As Object
        Dim par1 As String, par2 As String, expr As String


        Dim sr As StreamReader = New StreamReader("C:\Temp\leggi.txt")
        par1 = sr.ReadLine()
        par2 = sr.ReadLine()
        expr = sr.ReadLine()
        sr.Close()


        Code = "Function Calcolo()" & vbCrLf
        Code = Code & "m1 = " & par1 & vbCrLf
        Code = Code & "m2 = " & par2 & vbCrLf
        Code = Code & "Calcolo = " & expr & vbCrLf
        Code = Code & "End Function"


        VBScript.Language = "VBSCRIPT"
        VBScript.AllowUI = True
        VBScript.AddCode(Code)
        Result = VBScript.Run("calcolo")


        Dim sw As StreamWriter = New StreamWriter("C:\Temp\file.txt")
        sw.WriteLine(Result.ToString)
        sw.Close()


        VBScript = Nothing