Is there any mantra attributed to Vishnu in the Veda (moola samhita). Not mention of word "Vishnu" but is he a "devata" for any specific mantra for example "brahmanaspati" devata for "gana nan tva ganapati..."
As you can see in the Rig Veda Anukramani given in my answer here, Vishnu is the Devata of several hymns of the Rig Veda:
- Book 1 Hymn 22 Verses 16-21
- Book 1 Hymn 154
- Book 1 Hymn 155
- Book 1 Hymn 156
- Book 7 Hymn 99
- Book 7 Hymn 100
Also, there are several hymns of the Rig Veda where the Rishis is either Vishnu or one of his incarnations:
- Book 10 Hymn 90 was heard by Vishnu's incarnation sage Narayana.
- Book 10 Hymn 110 was heard by Vishnu's incarnation Parashurama
- Book 10 Hymn 134 was heard by Vishnu's incarnation Mandhata
- Book 10 Hymn 148 was heard by Vishnu's incarnation Prithu
- Book 10 Hymn 184 was heard by Vishnu himself
By the way, while the Devata may very from one hymn to another, all the hymns of the Vedas ultimately praise Vishnu. As Krishna says in the Bhagavad Gita, " Those who are devotees of other gods and who worship them with faith actually worship only Me."