Which Scriptures say Vritra was an Asura.
Markandeya Purana – Chapter 5 – Indra’s transformations.
Background – In this chapter we find the story of birth of Vritrasura , here he is called as asura. Here in this chapter a bird explaining about five Pandavas being incarnations of Indra.
Indra Killed Tvasta’s son , then hearing his son was slain , the ...
The reason Lord Tvastar created vajra for Indra is discribed here in tattireya samhita of yajur veda kanda 2.
Tvastr, his son slain, offered Soma excluding Indra. Indra desired an invitation to the rite, but he did not invite him, (saying), 'Thou hast slain my son.' He made a disturbance of the sacrifice, and forcibly drank the Soma. The remains ...
There is a lot of confusion regarding these two perhaps because the Puranas mention Vishwakarma as the doer of things that Vedas mention Tavshtar to have done. This book mentions the same:
But there is a huge distinction between Vedic & Puranic Vishwakarma - Vedas use Vishwakarman most often as an adjective not the name of a person. It is an epithet for ...
The Ribhus are the three sons of Angiras' son Sudhanvan. This chapter of the Aitareya Brahmana of the Rig Veda briefly describes their story:
[T]he Ribhus by fervor among the gods won the drinking of Soma. For them they desired to arrange it at the morning pressing; them Agni with the Vasus repelled from the morning pressing. For them they sought to ...
Rbhus are the Divine Artisans. Having attained to divinity by virtue of their action and knowledge, these deities work to help man to attain divinity.
I am quoting from the book RigVeda Mandala 1, R.L.Kashyap.
The RV (Rig Veda) attaches equal importance to all aspects of life; it
does not have rigid compartments of worldly life and spiritual life.
Yes, Vritra is described as an Asura in Mahabharata many times. I am giving below some of the verses.
32 अनायुषः पुनः पुत्राश चत्वारॊ ऽसुर पुङ्गवाः
विक्षरॊ बलवीरौ च वृत्रश चैव महासुरः
And Danayu also had four sons who were bulls among the Asuras. They were Vikshara, Vala, Vira, and Vritra the great Asura. Mahabharata, Adi Parva, SECTION LXV
The verses you shared do point to that conclusion but it seems unlikely that Maya Danav would be related to Vishwakarma since he is mentioned in the Mahabharata as a brother of Namuchi, an Asura who was killed by Indra. According to the Khandav Dahan Parva of Mahabharat:
"And it so happened that the slayer of Madhu suddenly beheld an Asura of the name of ...
The Rig Vedic Verse 10.82 mentions Tvashtar emanating from Vishwakarman but thats not all. There are different layers to this enigmatic god depending on which scriptures you read. The Vedic Tvashtar is described in various hymns as the artist who creates an embryo inside a woman's womb.
Atharva Veda Book 5 Hymn 25 mentions the following:
4-5 Let Mitra-...
Tvashta Deva is mentioned many times in Rigveda. Tvashta is a creator of universe emerged from or related to Vishvakarma.
From Purusha Sukta:
॥ उत्तरनारायणम् ॥
अद्भ्यः संभूतः पृथिव्यै रसाच्च । विश्वकर्मणः समवर्तताधि ।
तस्य त्वष्टा विदधद्रूपमेति । तत्पुरुषस्य विश्वमाजानमग्रे । १
English translation from Sri Vaishnava site:
Well the following verse from Rigved 1.32.9 mentions Vritra as the son of Danu:
8 There as he lies like a bank-bursting river, the waters taking courage flow above him.
The Dragon lies beneath the feet of torrents which Vṛtra with his greatness had encompassed.
9 Then humbled was the strength of Vṛtra's mother: Indra hath cast his deadly bolt against ...
Before getting to Trishiras I have to mention a little more about his father Tvashtar. here are various interpretations of who really Tvashtar was and you can check the evidence as shared in the answer to this question. According to Vishnu Puran Book 1 Chapter 25:
The sister of Váchaspati, lovely and virtuous, Yogasiddhá, who pervades the whole world ...