codice:
'in Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim f As New Form2
    f.Owner = Me
    f.ShowDialog()
End Sub

'sub richiamabile dall'esterno
Public Sub Procedura(ByVal txt As String)
   Console.WriteLine(txt)
End Sub

'in Form2
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Dim f As Form1

    f = Me.Owner

    f.Procedura("Prova")
End Sub