Acccording to Phala Deepika:
वर्गोत्तमांसस्थ दस सुभप्रदा !
मिस्रैवसा चास्तमिते च नीचगे !!
मृत्युव्ययारीस दसापहारयो !
स्तत्र स्थितस्याप्य शुभं फलं भवेतः !!
The planet which is vargottam always gives favourable results in its dasa. Such planet if in its Debilitation raasi or combust will give mixed results.But if the both the Dasa and antardasas are of Eigth and Twelth lords or the planets which are in Sixth,Eigth or Twelth houses then the planet gives unfavourable results.
