Rig Veda Book 10 Verse 48 refers to Indra as Indra Vaikuntha. From what I understand, Vaikuntha was also the name of an incarnation of Lord Vishnu in the Raivat Manvantar though the Indra then was someone else.
SO my question is - Is there a relation between the present Indra and Vaikuntha incarnation of Lord Vishnu? If not why is he referred to as Vaikuntha?
The devata of Rigveda tenth mandala 47 and 48 suktas (hymns) is Indra Vaikuntha. This is not Vishnu but one of the incarnations of Lord Indra. He is named as Vaikuntha because of his mother Vikuntha, daughter of Prajapati.
There is a text called Bṛhaddevatā which is written by Sage Shaunaka. This text contains the details of the deities worshipped in hymns of Rigveda. It also gives the background stories of composition of the mantras.
There is explanation given for these hymns.
There was an Āsuri, daughter of Prajāpati, Vikunthā by name. She, desiring a son like Indra, performed very severe austerities. She then obtained from Prajāti her desires (in the form of) various boons. And Indra himself was born of her, as he wished to slay the Daityas and Danavas. Once he was engaged in battle with the Danavas. Of them he slew nine nineties and seven groups of seven. Having shattered with the might of his arm the citadels of gold, silver, and iron, (and) having slain all (of them) in their respective spheres (yathāsthānam), as arrayed on earth and the other (two worlds). On earth he exterminated both the Kālakeyas and the race of Puloma, the archer, and in heaven the notorious (tan) offspring of Prahlada.
Having obtained sovereignty among the Daityas (and) puffed up with pride by reason of his might, he began to harass the gods, being infatuated by the craft of the Asuras. Now while they were being harassed by that same Asura of unlimited power, they fled for succour to Saptagu, most excellent of seers, in order that the latter should admonish him (Indra). Now the seer called Saptagu was a dear friend of his, and (so) he praised him with the (hymn), 'We have grasped' (x.47) as he took him be the hand. Then he coming to (buddhva) himself (and) rejoiced at the praise of Saptagu, praised himself with the three (hymns) 'I was' (aham bhuvam: x.48-50)[.]
Emphasis mine. Quoted portion taken from a question by Keshav Srinivasan.
Indra Vaikuntha's story in the text is concluded in chapter 7 story 13. The next verses recall his deeds from past.
If we observe, the incidents given in the passage, can be related to some incidents given in epics and puranas. "In the heavens, the notorious..." reminds us of Indra slaying Virochana. The linked question also gives a verse from Kaushitaki Upanishad where Indra says he slain people of Prahlâda in heaven, the people of Puloma in the sky, the people of Kâlakañga on earth which is in relation with the story given in Bṛhaddevatā. It also matches with the seers (rishis) of the sukta. According to Rigvedic Anukramani, rishi and devata of 48-50 suktas is Indra Vaikuntha.
So, it is Indra who is called Indra Vaikuntha in the aforementioned suktas and this name has no relation with Lord Vishnu.
The Devata of Rig-Veda Mandala 10 - Sukta 47-50. Is Indra-Vaikuntha. And here in these suktas Indra is called as Vaikuntha. And why he is called so ? The possible answer is found in Shreemad Bhagavad Purana - Skanda 8- Chapter 5- Verse 4. Here the purana tells us that Vaikuntha is name of one of the incarnations of Lord Vishnu as well as Vaikunthas are class of special gods. , who are his expansions. Below is the verse.
पत्नी विकुण्ठा शुभ्रस्य वैकुण्ठैः सुरसत्तमैः ।
तयोः स्वकलया जज्ञे वैकुण्ठो भगवान्स्वयम् ॥ SB 8.5.4॥patnī vikuṇṭhā śubhrasya vaikuṇṭhaiḥ sura-sattamaiḥ
tayoḥ sva-kalayā jajñe vaikuṇṭho bhagavān svayamVaikuntha was wife of Subhra ; with a portion of his energy ,Lord Vaikuntha was born of these of his own accord , along with great gods known as Vaikunthas.
So from above explanation we possibly can say that in in these suktas of Rig-Veda Indra is equated with these special gods who are expansions of Lord vishnu for his eulogy purpose. We can also see that in Rig-veda and other Vedas deities are often praised by various names and in various forms i.e. deities are often equated with Brahman and respected deities are shown as supreme ones.
So I think Indra is praised here by equating him special class of gods who are forms of Lord Vishnu and are called as Vaikunthas and hence he is called in Rig-Veda as Indra-Vaikuntha.
