Is the vedic supreme God different from the puranic supreme God?
If not, then why is the puranic concept of God trimurty and vedic concept, brahman?
Summary: the fundamental essence of Brahman is the same in the Vedas and Puranas. The Puranas are only a simplified version to take people to the same Supreme God. The same Brahman is extolled with attributes (Saguna) in the Puranas.
Detailed: In the Vedas and Puranas, the essence of the Supreme God is the same. the Supreme God is Brahman, the omnipotent, omnipresent and omniscient lord of the sentient (चेतन) and insentient (जड़). All scriptures are ultimately meant for realising, praising and finally going to the Supreme God. The scriptures are to be read in conjunction with each other.
The chhanda of Ramcharitmanas 1.51 says that both, the Vedas and Puranas, praise the Supreme God:
मुनि धीर जोगी सिद्ध संतत बिमल मन जेहि ध्यावहीं। कहि नेति निगम पुरान आगम जासु कीरति गावहीं॥ सोइ रामु ब्यापक ब्रह्म भुवन निकाय पति माया धनी। अवतरेउ अपने भगत हित निजतंत्र नित रघुकुलमनि॥
Whom the sages, yogis, siddhas always meditate upon with a pure mind. Whose praise is sung by the Vedas Shastras and Puranas by saying ‘not this, not this’; that Rama who is the Supreme Brahman, the ever independent lord of the entire universe and Maya, has manifested as the gem of the Raghu dynasty for the sake of his devotees.
As far as the Vedas go there are many answers already describing the Vedic essence of Brahman. In this answer you can find a description about Trimurtis representing Brahman. Yet a few verses from the Vedantas:
ईशावास्यमिदं सर्वं यत्किंच जगत्यां जगत। तेनत्येक्तेन भुञ्जीथा मा गृधः कस्य स्विद्धनम्॥१॥
Whatever is the root-consciousnesses in this creation is all of God. Use only that given by him do not covet more; because whose wealth is it? It belongs to no one but only to God. - Ishavasyopanishad verse 1
केनेषितं पतति प्रेषितं मनः। केन प्राणः प्रथमः प्रैति युक्तः। केनेषितां वाचमिमां वदन्ति। चक्षुः श्रोत्रं क उ देवो युनक्ति॥
by whom does this mind, which is transmitted, fall on its target? By whom does the first life appointed proceed on its path? Who is inspired by whom humans speak? Who is the god who appointed eyes and nose in their actions? - Kenopanishad verse 1
The Puranas are meant for a more simpler understanding of the same essence of the Supreme God. In the Puranas, a greater emphasis has been placed on worshipping of the Saguna (one with form and attributes) form of the Lord. Each Purana glorifies the deity to which it is dedicated, as Brahman and Supreme to the Saguna forms of the Trimurti. That is the deity among the Trimurti that the Purana is dedicated to, is glorified as the Supreme. For example:
- The Vishnu Purana and Srimad Bhagavata Purana are dedicated to Lord Vishnu and hence Vishnu is glorified, i.e. Brahman in his Saguna form of Vishnu is glorified as Brahman. His description is that of Vishnu and yet he assumes the form of Brahma Shiva and again Vishnu (Saguna).
Śrī, the bride of Viṣṇu, the mother of the world, is eternal, imperishable; in like manner as he is all-pervading, so also is she, oh best of Brahmans, omnipresent. - Vishnu Purana Chapter 8
Hail to thee, who art all creatures; to thee, the holder of the mace and shell ... Hail to thee, spirit of the supreme spirit; to thee, soul of soul; to thee, who art discrete and indiscrete matter; who art one with the elements and with time. Thou art the creator of all things, their preserver, and their destroyer, in the forms, oh lord, of Brahmā, Viṣṇu, and Rudra, at the seasons of creation, duration, and dissolution. - Vishnu Purana Chapter 4
The transcendental Personality of Godhead is indirectly associated with the three modes of material nature, namely passion, goodness and ignorance, and just for the material world’s creation, maintenance and destruction He accepts the three qualitative forms of Brahmā, Viṣṇu and Śiva. Of these three, all human beings can derive ultimate benefit from Viṣṇu, the form of the quality of goodness. ŚB 1.2.23
Being Puranas dedicated to Bhagwan Vishnu, the essence of these is that the Saguna form of Brahman (which assumes all the trimurtis forms) which is Vishnu is the one which represents Brahman most accurately and is the best form to worship.
- The Shiva Purana is dedicated to Shiva and hence it is he who is eulogised as the best form of Brahman.
Although Nirguṇa, I am Saguṇa too and the author of dissolution, maintenance and creation. I am the supreme Brahman without decay and change. Existence, Knowledge and Bliss are my characteristics. Truly, I am Niṣkala (Nirguṇa) for ever, O Hari. For the activities of creation, maintenance and dissolution I manifest myself in the three forms of Brahmā, Viṣṇu and Hara (verse 27-28)
A great form similar to this, O Brahmā, shall become manifest in the world through your body. He will be called Rudra. His capacity will never he less, since He will be my own part and parcel. He is I. I am he. In the modes of worship too there in no difference (verse 30-31) - Shiva Purana, Chapter 9 of Rudra-saṃhitā Sṛṣṭi-khaṇḍa
Here again we can see that Brahman says he is Nirguna and assumes the Saguna form of the Trimurti. Being a Shaiva Purana, the Saguna form of Shiva (Rudra) is said to be the most accurate representation of Brahman.
- The Devi Bhagavata obviously extols Devi as Brahman:
O Auspicious Mother! It is Thou in the shape of Brahmâ, Visnu and S’iva, That art creating this Universe and it is Thou that hast assumed the form of this whole Universe, moving and non-moving. Thus Thou playest, as it wills Thee, under various forms, again and again. Thou dost cease from play (during pralaya) as it likes Thee. - Chapter 5, 3rd Skandha of the Devi Bhagavata
Puranas describe the particular ishta deva of that sect as Brahman, and say that Brahman in the form of that ishta deva is the best way to reach the ultimate supreme reality. By Brahman manifesting as the Trimurti, non-yogis too can get an image on whom to meditate/ praise, etc. and are able to better associate with the Supreme God.
Thus from the above, we can see that the essence (there maybe other differences but the fundamental aspect is the same) of the Supreme God is the same in the Vedas and Puranas. Furthermore the Trimurti are nothing but manifestations of the same Supreme God. It depends on one’s own choice whom he wants to worship and which Purana is selected. For example, for people who adore Lord Vishnu, there can be nothing beyond him, he is avyakta, etc. None of the views are wrong and all are absolutely correct.
Also refer: Supreme Brahman in Hinduism
According to Vedas and upanishads, supereme parama barahma who is shapeless and formless take form as Brahma, Vishnu and Shiva. For the process of creation, preservation and destruction.
Kaivalya Upanishad mantra 8
He is Brahma, he is Siva, he is Indra. He is the imperishable, supreme self illumined Lord. He alone is Vishnu. He is the life giving breath. He is time, he is fire, and also the moon.
Same thing is written in Vishnu Purana
Thus the one only god, Janárddana, takes the designation of Brahmá, Vishńu, and Śiva, accordingly as he creates, preserves, or destroys."
The conclusion is that both Puranas and Vedas talks about same parambrhama who take form of Brahma, Vishnu and Mahesh.