https://www.wisdomlib.org/hinduism/book/the-ramayana-of-valmiki/d/doc424859.html
“It is said in the Puranas that the magnanimous Vasava, sullied by the sin of brahmanicide, was cleansed by performing the Horse-sacrifice. O Long-armed Warrior, in former times when Devas and Asuras were united, there lived a universally honoured Daitya named Vritra. The width of his body was an hundred leagues and he was three times as tall. In his loving-kindness, he cast his beneficent glance in all directions on the Three Worlds. Loyal, grateful, highly intelligent, he ruled his fertile territory with care and integrity and, under his dominion, the earth produced all that could be desired—flowers, roots and delicious fruits. Without being cultivated the earth was abundantly fruitful and for many years that magnanimous prince enjoyed a rich empire marvellous to behold.
Then the thought came to him:
‘I shall perform a rigid penance; in truth, asceticism is a great joy, all other happiness is a mere illusion.’
“Having established his eldest son over his people as King of Madhura, he gave himself up to a rigid penance that caused terror among the Gods.
As Vritra was mortifying himself thus, Vasava, in his extreme affliction, sought out Vishnu and spoke to him as follows:—
“‘On account of his asceticism that long-armed hero, Vritra, has conquered the worlds; he is powerful and virtuous; I shall not be able to overcome him. If he continues with these austerities, O Chief of the Gods, we shall be subject to him as long as the worlds endure. You have overlooked the extremely illustrious Vritra or he would not live an instant in the face of your wrath, O Lord of the Gods. From the moment he succeeded in propitiating you, O Vishnu, he has taken over the direction of the worlds. It is for you in your great solicitude to befriend the universe, then by your favour, the worlds will live in peace, free from affliction. All the inhabitants of the celestial region have fixed their gaze on you, O Vishnu. Slay Vritra and by this great feat deliver them! You have ever lent support to the magnanimous Gods which cannot be withstood by their adversaries; be the refuge of those who have no other refuge!’”
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“Such was the entreaty addressed by all the inhabitants of heaven led by that Lord of a Thousand Eyes, to Vishnu, who answered all those Gods with Indra at their head, saying:—
“‘An ancient tie binds me to the magnanimous Vritra, therefore I am unable to favour you by slaying that great Asura; it is not possible for me to grant you that supreme felicity but I will indicate the means whereby the Thousand-eyed God may destroy him. I shall divide my natural essence into three parts, O Foremost of the Gods, and by these means, that God of a Thousand Eyes can undoubtedly slay Vritra. A third of my being will enter Vasava, a second part into the thunderbolt and a third will enter the bosom of the earth, thus Vritra will perish!’
According to the theory that Indra is only a post - why did Vishnu favor Indra over Vrittra?
