The Prajāpati of the Vedic scriptures is NOT the mythological Brahmā of the Epic-Puranic literature. This is evident from the explicit statement in Vājasaneyi Saṃhitā (32.5) which states that Prajāpati is the God before whom nothing was born and who pervaded all the worlds.
यस्माज् जातं न पुरा किं चनैव य आबभूव भुवनानि विश्वा ।
प्रजापतिः प्रजया सम्रराणस् त्रीणि ज्योतीम्षि सचते स षोडशी ॥
This pervasiveness of the Prajapati is also explicated by Taittrīya Saṃhitā, which describes Prajāpati as unlimited (TS 1.7.3.2).
अ॒न्वाह॑रति॒ तद॑न्वाहा॒र्य॑स्यान्वाहार्य॒त्वं दे॑वदू॒ता वा ए॒ते यदृ॒त्विजो॒ यद॑न्वाहा॒र्य॑मा॒हर॑ति देवदू॒ताने॒व प्री॑णाति प्र॒जाप॑तिर्दे॒वेभ्यो॑ य॒ज्ञान् व्यादि॑श॒त् स रि॑रिचा॒नो॑ऽमन्यत॒ स ए॒तम॑न्वाहा॒र्य॑मभ॑क्तमपश्य॒त् तमा॒त्मन्न॑धत्त॒ स वा ए॒ष प्रा॑जाप॒त्यो यद॑न्वाहा॒र्यो॑ यस्यै॒वं वि॒दुषोऽन्वाहा॒र्य॑ आह्रि॒यते॑ सा॒क्षादे॒व प्र॒जाप॑तिमृध्नो॒त्यप॑रिमितो नि॒रुप्योऽप॑रिमितः प्र॒जाप॑तिः प्र॒जाप॑तेः
The Śatapatha Brāhmaṇa also leaves the reader without any doubt by exclaiming that Prajāpati was alone in the beginning (SB 2.2.4.1).
प्रजापतिर्ह वा इदमग्र एक एवास । स ऐक्षत कथं नु प्रजायेयेति सोऽश्राम्यत्स तपोऽतप्यत सोऽग्निमेव मुखाज्जनयांचक्रे तद्यदेनं मुखादजनयत तस्मादन्नादोऽग्निः स यो हैवमेतमग्निमन्नादं वेदान्नादो हैव भवति
It is the Prajāpati that created the earth, the sky and heavens by uttering bhūḥ, bhuvaḥ and svaḥ (SB 11.1.6.3).
स संवत्सरे व्याजिहीर्षत्। स भूरिति व्याहरत्सेयं पृथिव्यभवद्भुव इति तदिदमन्तरिक्षमभवत्स्वरिति साऽसौ द्यौरभवत्तस्मादु संवत्सर एव कुमारो व्याजिहीर्षति संवत्सरे हि प्रजापतिर्व्याहरत्
It is the Prajāpati that created all the living beings by assuming the form of Tortoise (SB 7.5.1.5).
स यत्कूर्मो नाम । एतद्वै रूपं कृत्वा प्रजापतिः प्रजा असृजत यदसृजताकरोत्तद्यदकरोत्तस्मात्कूर्मः कश्यपो वै कूर्मस्तस्मादाहुः सर्वाः प्रजाः काश्यप्य इति
According to Taittrīya Brāhmaṇa, it is the Prajāpati who assumed the form of the Boar and rescued the earth plunged in the water (TB 1.1.3.5).
अबधिरो भवति । य एवं वेद । प्रजापतिः प्रजा असृजत । तासाम-न्नमुपाक्षीयत । ताभ्यः सूदमुप प्राभिनत् । ततो वै तासामन्नं नाक्षीयत । यस्य सूदः संभारो भवति । नास्य गृहेऽन्नं क्षीयते । आपो वा इदमग्रे सलिलमासीत् । तेन प्रजापतिरश्राम्यत्
The importance of the Prajāpati can be gauged from the fact that Nirukta states that those hymns in which no deity is indicated are to be understood as dedicated to Prajāpati (Nirkuta 7.4).
अथ अन्यत्र यज्ञात् प्राजापत्या इति याज्ञिकाः ।तत् ये अनादिष्टदेवता मन्त्राः तेषु देवतोपपरीक्षा ।यद्देवतः सः यज्ञः वा यज्ञाङ्गं वा तद्देवता भवन्ति ।
Yāska the famed author of the Nirukta has placed Prajāpati as the god of the middle region (madhyasthānādevtā) appearance. The etymological meaning of the term Prajāpati is a protector or supporter of living beings. The word Prajāpati is formed from the root pā or pāl (to protect) and it is added to the normal stem prajā to form the word Prajāpati (10.42).
प्रजापतिः प्रजानां पाता वा पालयिता वा ।
In Nighaṇṭu (5.4), many gods are grouped together because of the similar attributes that they share with Prajāpati. These appellations in Nirukta include Viśvakarman (10.25 विश्वकर्मा सर्वस्य कर्ता । तस्य एषा भवति ।), Dhātṛ(11.10 धाता सर्वस्य विधाता ), Brahmaṇaspati (10.12 पालयिता वा ।ब्रह्मणस्पतिः ब्रह्मणः पाता वा ।), Bṛhaspati (10.11 बृहस्पतिः विरवेणा विकृत्य), Kṣetrasyapati (10.14 क्षेत्रस्य पतिः ।), Vāstoṣpati (10.16 तस्य पाता वा पालयिता वा ।वास्तुः वसतेः निवासकर्मणः । वास्तोष्पतिः ।), Vācaspati (10.17 वाचस्पतिः वाचः पाता वा पालयिता वा । ), Parjanya (10.10), Yama (10.19 यमः यच्छति इति सतः ।), and Mitra (10.21).
The Śatapatha Brāhmaṇa further states that Prajāpati is Vācaspati (5.1.1.16 प्रजापतिर्वै वाचस्पति), Viśvakarman (7.4.2.5 प्रजापतिर्वै विश्वकर्मा), Dhātṛ (9.5.1.38 प्रजापतिर्धाताथ), Agni (2.3.3.18 प्रजापतिर्वा अग्निः). The Taittrīya Brāhmaṇa expands this list further to include that Prajapati is ka (2.2.10.1 को ह वै नाम प्रजापतिः । य एवं वेद), Vāc (1.3.4.5 प्रजापतिर्हि वाक्). The Tāṇḍya Mahabrāhṃana (16.5.17 प्रजापतिर्वै सविता) further states that Prajapati Savitṛ. The Aitareya Brāhmaṇa has accorded the epithet of Vāyu to Prajāpati (4.26 वायुर्ह्येव प्रजापति). Presumably, because of all these appellations, Shaunaka in Bṛhaddevatā says that the names of all the deities can be attributed to Prajapati as he is the source of all of them (3.72).
In the Ṛg Veda, one finds very few mantras the attributed to Prajāpati. It occurs as an appellation of Savitṛ, Pūṣan and Soma (RV 4.53.2, 9.5.9)
दिवो धर्ता भुवनस्य प्रजापतिः पिशङ्गं द्रापिं प्रति मुञ्चते कविः ।विचक्षणः प्रथयन्नापृणन्नुर्वजीजनत्सविता सुम्नमुक्थ्यम्
त्वष्टारमग्रजां गोपां पुरोयावानमा हुवे ।इन्दुरिन्द्रो वृषा हरिः पवमानः प्रजापतिः
and is beseeched along with Viṣṇu, Tvaṣṭṛ and Dhātṛ for children (10.184.1).
विष्णुर्योनिं कल्पयतु त्वष्टा रूपाणि पिंशतु ।आ सिञ्चतु प्रजापतिर्धाता गर्भं दधातु ते
The famous Hiraṇyagarbha Sukta (10. 121.10) is a prayer to Hiraṇyagarbha but ends mentioning Prajāpati. The Śatapatha Brāhmaṇa also supports the statement (SB 6.2.2.5 प्रजापतिर्वै हिरण्यगर्भः) that Prajāpati is Hiraṇyagarbha.
The word Prajāpati occurs in the list of Nighaṇṭu as a hint to sacrifice (3.17). Prajāpati is sacrificed is the commonly occurring theme in the Brāhmaṇa scriptures (Aitareya Br. 2.17, Śatapatha Br. 1.1.1 13 etc. etc.). This is confirmed when Prajāpati desires to offer his own self because he is non-different for sacrifice (Sāyaṇa’s commentary to Śatapatha Br. 2.2.4.4). A Vedic reader will note the similarity with Puruṣa Sukta (RV 10.90), wherein the Puruṣa sacrifices himself. This interpretation is further strengthened by the utterances of Śatapatha Brāhmaṇa which reiterates that the Puruṣa became Prajapati (SB 6.1.1.5).
स एव पुरुषः प्रजापतिरभवत् । स यः स पुरुषः प्रजापतिरभवदयमेव स योऽयमग्निश्चीयते
Prajāpati is restored by Agni who gives up his own body (the fire-altar) to create new Prajāpati from the mutilated parts thereof (SB 6.1.2.27)
उभयं हैतद्भवति । पिता च पुत्रश्च प्रजापतिश्चाग्निश्चाग्निश्च प्रजापतिश्च प्रजापतिश्च देवाश्च प्रजापतिश्च य एवं वेद
Hence, Prajāpati of the Vedic scriptures cannot be relegated to the Brahmā of the Puranic and Epic literature.