Vishwakarman Prajapati is the Supreme cosmic creator god also known as Swayambhu Brahman (self existing) or Narayana/Purusha in the vedic texts. He is the father of the gods and the asuras. He is the highest Brahman (Para Brahman) and called as ‘the Lord of all existence’ in Satapatha Brahmana.
He is ‘Aja’ (Unborn) and as mentioned in Vishwakarma suktam of Yajurveda, he created ‘Brahmanda’ (where the universe exists) containing all the Gods and the Asuras which lies connected to his navel by means of a receptacle. This picturisation of Vishwakarman Prajapati was later transferred to Vishnu by Vyasa in his Puranas and Mahabharata and called him ‘Padmanabha’and ‘Sheshasayana’.
Incarnations like Varaha(Yajurveda Taittiriya Samhita - 7.1.5, Taittiriya Brahmana - 1.1.3.5, Kapisthala Samhita - 6.7, Kathaka Samhita - 8.2, Atharvaveda - 12.1.60)
Ramayana - 2.110.3,4 :- “Swayambhu Brahman assuming the form of a boar, caused the earth to rise from water.”
Matsya and Kurma were attributed to Vishwakarman Prajapati in the Vedic texts (Satapatha Brahmana - 1.8.1-6 and Satapatha Brahmana - 7.5.1.5) but were later transferred to Vishnu by Vyasa.
The Purusha suktam of Shukla Yajurveda and Taittiriya Aranyaka clearly states that Vishwakarman Prajapati is Purusha. Nowhere in the suktam is mentioned the word Vishnu.
Vyasa replaced Vishwakarman Prajapati with Vishnu as the Supreme god in all of his written scriptures.
For references :-
The Development Of Hindu Iconography
By Jitendra nath banerjea 1941 pages 301 and 302
and
Anantasayana Vishnu: Earliest Epigraphic Evidence
by Ajay Mitra Sastri-
Talking about Adi sayana/Ananta sayana/Sesha Sayana form of Vishnu, Sastri wrote -
“There is no doubt, however, that the concept underlying this icon goes back to a much earlier period, though it was not associated with Vishnu. It is actually a mythological elaboration of the Rigveda - 10.82.5-6, where the original principle, there called Vishwakarman, is said to have been ‘beyond the sky, beyond the earth, beyond the gods and the Asuras, the earliest embryo the waters contained wherein all the divinities resided.’ In the Manu Smriti - 1.10 and the Mahabharata - 12.341 a slightly altered version of the same myth is applied to Prajapati. We are told that the waters were known as Nāras as they were sons of Nara, and they were the abode of Prajapati, he came to be known as Narayana. The only thing that was required for regarding it as a form of Vishnu was just to transfer this legend from Vishwakarman/Prajapati to god Vishnu after he came to be looked upon as the supreme god by his votaries and adjust it to the Vaishnava mythology with a view to give it an iconic form.”
Vedic Mythology
by A. A. Macdonell (page - 41)-
Talking about the Varaha form of Vishwakarman Prajapati, he says -
“In the post-Vedic mythology of the Ramayana and the Puranas, the boar which raises the earth, has become one of the Avatars of Vishnu.
The germs of two other Avatars of Vishnu are to be found in the Brahmanas, but not as yet connected with Vishnu. The fish which in the Satapatha Brahmana - 1.8.1.1 delivers Manu from the flood, appears in the Mahabharata as a form of Prajapati, becoming in the Puranas an incarnation of Vishnu. In the Satapatha Brahmana - 7.5.1.5 and Taittiriya Aranyaka - 1.23, Prajapati about to create offspring becomes a tortoise moving in the primeval waters. In the Puranas this tortoise is an Avatar of Vishnu.”
Annals of The Bhandarkar Oriental Research Institute LXXV 1994 (page - 112)-
“Originally this myth also was associated with Prajapati. The Satapatha Brahmana ( 14. 1. 2 ) tells us that Prajapati in the form of a boar raised the earth from the bottom of the ocean. The Taittiriya Aranyaka represents the earth as having been rescued from the waters by a hundred-armed black boar. Only this function was transferred from Prajapati, who was earlier regarded as the supreme divinity, to Vishnu after the latter replaced him as the supreme god.”
Encyclopedia of Ancient Deities
By Charles Russell Coulter & Patricia Turner
Page 281-
“In the Satapatha Brahmana, Kurma is an avatar of Prajapati. Some records indicate that many of Prajapati's exploits were tranferred to Vishnu.”