"Brahman" is mentioned in Vedas. Not "Brahmā", the deva.
While Brahman may be mentioned, I believe that Brahmā is (albeit in a different name) within the Vedas; at the very least, the seed of his existence was there. That is, unless this is a reference to Brahmā like Ralph Griffiths believed:
(AVŚ_19,42.1a) bráhma hótā bráhma yajñā́ bráhmaṇā sváravo mitā́ḥ |
(AVŚ_19,42.1c) adhvaryúr bráhmaṇo jātó bráhmaṇo 'ntárhitaṃ havíḥ ||1||
(AVŚ_19,42.2a) bráhma srúco ghr̥távatīr bráhmaṇā védir úddhitā |
(AVŚ_19,42.2c) bráhma yajñásya táttvaṃ ca r̥tvíjo yé haviṣkŕ̥taḥ |
(AVŚ_19,42.2e) śamitā́ya svā́hā ||2||
(AVŚ_19,42.3a) aṃhomúce prá bhare manīṣā́m ā́ sutrā́vṇe sumatím āvr̥ṇānáḥ |
(AVŚ_19,42.3c) imám indra práti havyáṃ gr̥bhā́ya satyā́ḥ santu yájamānasya kā́māḥ ||3||
(AVŚ_19,42.4a) aṃhomúcaṃ vrṣabháṃ yajñíyānāṃ virā́jantaṃ prathamám adhvarā́ṇam |
(AVŚ_19,42.4c) apā́ṃ nápātam aśvínā huve dhíya indriyéṇa ta indriyáṃ dattam ójaḥ ||4||
- Atharva Veda, 19:42
1Brahma is Hotar, sacrifice: with Brahma are the stakes set up.
From Brahma was the Adhvaryu born, from Brahma hidden
offering.
2Brahma is fatness-dropping scoops: with Brahma was the altar
reared. p. 245
Brahma is worship, lengthened rite, the Rishis who pay sacrifice,
the victim's Immolators. Hail!
3To him who frees from woe mine hymn I offer, to the Good
Guardian, as I seek his favour.
Accept this offering of mine, O Indra. Fulfilled be all the sacri-
ficer's wishes!
4With prayer I call on him who frees from trouble, Prince of
Gods, Splendid, chief of sacrifices,
I call the Waters' Child and both the Asvins, Vigour is mine,
and strength bestowed by Indra.
Atharva Veda: Book 19: Hymn 42: In praise of Brahma, Prayer, or Devotion
Do you not take the hymns of Viśvakarman,
Prajāpati, identified with Hiraṇyagarbha as being in reference to Brahmā deva?
Do you disagree with this quote, which identifies Brahmā with Puruṣa and Brahmaṇaspati/Bṛhaspati?
Brahmanaspati and Brihaspati of the Vedas and the Brahma of the Hindu Trinty are said to be one and the same god. He is also the Purusha of the early Rigvedic hymns, the product of sacrifice, from whom emerge the worlds, the beings and the social order.
By the way, when I say a Vedic deity, I didn't necessarily mean within the Vedic scripture, as such, but within the Vedic society timeline.