Śrīla Viśvanātha Cakravartī Ṭhākura explains that Indra, not Vṛtrāsura, was actually killed. He says that when Vṛtrāsura swallowed King Indra and his carrier, the elephant, he thought, “Now I have killed Indra, and therefore there is no more need of fighting. Now let me return home, back to Godhead.” Thus he stopped all his bodily activities and became situated in trance. Taking advantage of the silence of Vṛtrāsura’s body, Indra pierced the demon’s abdomen, and because of Vṛtrāsura’s trance, Indra was able to come out. Now, Vṛtrāsura was in yoga-samadhi, and therefore although King Indra wanted to cut his throat, the demon’s neck was so stiff that Indra’s thunderbolt took 360 days to cut it to pieces. Actually it was the body left by Vṛtrāsura that was cut to pieces by Indra; Vṛtrāsura himself was not killed. In his original consciousness, Vṛtrāsura returned home, back to Godhead, to become an associate of Lord Saṅkarṣaṇa. Here the word alokam means the transcendental world, Vaikuṇṭhaloka, where Saṅkarṣaṇa eternally resides.

Thus end the Bhaktivedanta purports of the Sixth Canto, Twelfth Chapter, of the Śrīmad-Bhāgavatam, entitled “Vṛtrāsura’s Glorious Death.”