As stated in Brahma-saṁhitā (5.43), all the Vaikuṇṭha planets in the spiritual sky (known as Viṣṇuloka) are situated in the planet known as Kṛṣṇaloka, Goloka Vṛndāvana. In that supreme planet the Lord enjoys His transcendental bliss in multiple forms, and all the opulences of the Vaikuṇṭhas are fully displayed in that one planet. The associates of Kṛṣṇa are also full with six opulences. In the Pādmottara-khaṇḍa (225.57) it is stated that the material energy and the spiritual energy are separated by water known as the Virajā River. That river flows from the perspiration of the first puruṣa incarnation. On one bank of the Virajā is the eternal nature, unlimited and all-blissful, called the spiritual sky, and this is the spiritual kingdom, or the kingdom of God. The spiritual planets are called Vaikuṇṭhas because there is no lamentation or fear there; everything is eternal.
Viraja is a river that divides the material world from the spiritual world
SB Canto 2
In the Padma Purāṇa, Uttara-khaṇḍa, it is stated that beyond the one-fourth part of God's creation is the three-fourths manifestation. The marginal line between the material manifestation and the spiritual manifestation is the Virajā River, and beyond the Virajā, which is a transcendental current flowing from the perspiration of the body of the Lord, there is the three-fourths manifestation of God's creation. This part is eternal, everlasting, without deterioration, and unlimited, and it contains the highest perfectional stage of living conditions. In the Sāṅkhya-kaumudī it is stated that unalloyed goodness or transcendence is just opposite to the material modes. All living entities there are eternally associated without any break, and the Lord is the chief and prime entity. In the Āgama Purāṇas also, the transcendental abode is described as follows: The associated members there are free to go everywhere within the creation of the Lord, and there is no limit to such creation, particularly in the region of the three-fourths magnitude. Since the nature of that region is unlimited, there is no history of such association, nor is there end of it.
After hearing Rāmānanda Rāya speak in this way, Lord Śrī Caitanya Mahāprabhu again rejected his statement and said, "Go ahead and say something more."
Rāmānanda Rāya then replied, "Devotional service mixed with empiric knowledge is the essence of perfection."
Devotional service mixed with non-Vedic speculative knowledge is certainly not pure devotional service. Therefore Śrīla Bhaktisiddhānta Sarasvatī in his Anubhāṣya preaches that self-realization following the execution of ritualistic ceremonies is in the neutral stage between liberation and conditioned life. It is a place beyond this material world, in the river Virajā, where the three modes of material nature are subdued or neutralized in the unmanifest stage. However, the spiritual world is a manifestation of spiritual energy and is known as Vaikuṇṭhaloka, "the place where there is no anxiety." The material world, known as brahmāṇḍa, is the creation of the external energy. Between the two creations—the material creation and the spiritual creation—is a river known as Virajā, as well as a place known as Brahmaloka. Virajā-nadī and Brahmaloka are shelters for living entities disgusted with material life and inclined to impersonal existence by way of denying material variegatedness. Since these places are not situated in the Vaikunṭḥalokas, or the spiritual world, Śrī Caitanya Mahāprabhu proclaims them to be external. In the Brahmaloka and Virajā-nadī, one cannot conceive of the Vaikuṇṭhalokas. Brahmaloka and Virajā-nadī are also attained after difficult austerities, but in these realms there is no understanding of the Supreme Personality of Godhead and His transcendental loving service. Without such spiritual knowledge, simple detachment from material conditions is but another side of material existence. From the spiritual point of view, it is all external.
“Just as there are millions of fruits on the uḍumbara tree, millions of universes float on the waters of the river Virajā.
Virajā is a river that divides the material world from the spiritual world. On one side of the river Virajā is the effulgence of Brahmaloka and innumerable Vaikuṇṭha planets, and on the other side is this material world. It is to be understood that this side of the Virajā River is filled with material planets floating in the Causal Ocean. The name Virajā indicates a marginal position between the spiritual and material worlds, but the Virajā River is not under the control of the material energy. Consequently it is devoid of the three guṇas.
“As one waters the bhakti-latā-bīja, the seed sprouts, and the creeper gradually grows to the point where it penetrates the walls of this universe and goes beyond the Virajā River, lying between the spiritual world and the material world. It attains brahma-loka, the Brahman effulgence, and penetrating through that stratum, it reaches the spiritual sky and the spiritual planet Goloka Vṛndāvana.
“That original Personality of Godhead, named Saṅkarṣaṇa, first lies down in the river Virajā, which serves as a border between the material and the spiritual world. As Kāraṇābdhiśāyī Viṣṇu, He is the original cause of the material creation.
“"Between the spiritual and the material world is a body of water known as the river Virajā. This water is generated from the bodily perspiration of the Supreme Personality of Godhead, who is known as Vedāṅga. Thus the river flows."
“"Beyond the river Virajā is the spiritual nature, which is indestructible, eternal, inexhaustible and unlimited. It is the supreme abode, consisting of three fourths of the Lord"s opulences. It is known as paravyoma, the spiritual sky.'
In the spiritual sky there is neither anxiety nor fear. It is eternally existing, and it consists of three fourths of the Lord's energy. The material world is an exhibition of only one fourth of the Lord's energy. Therefore it is called eka-pād-vibhūti.
“On the other side of the river Virajā is the external abode, which is full of unlimited universes, each containing unlimited atmospheres.
“‘“Beyond the river Virajā is the spiritual nature, which is indestructible, eternal, inexhaustible and unlimited. It is the supreme abode, consisting of three fourths of the Lord's opulences. It is known as paravyoma, the spiritual sky.”'
This is a verse from the Padma Purāṇa, quoted here by Lord Kṛṣṇa.