As far as I can tell, in the Rig Veda, Indra,Rudra Agni Soma have been extolled as the mightiest, separately (what westerners call henotheism). It is not even clear if Vishnu in the rig veda is depicted anthropomorphically. This question is specific to Rig Veda.
Yes, Lord Vishnu is also said supreme God in Rig veda
Who verily alone upholds the threefold, the earth, the heaven, and all living creatures.(Rig veda book 1 hymn 154)
In this hymn he has said alone to uphold creatures and the earth and heaven.
We laud this manly power of him the Mighty One, preserver, inoffensive, bounteous and benign;(Rig veda hymn 155 mantra 4)
Here in this verse Lord Vishnu is said to be Mighty,one preserver inoffensive.
5 With offerings I propitiate the branches of this swift-moving God, the bounteous Viṣṇu.
Hence Rudra gained his Rudra-strength:(Rig Veda book 7 hymn 40 verse 5)
Here Lord Rudra is said to gain strength from Lord Vishnu. In Rig Veda many deities are praised as Supreme as Vedas praises 1 God(Brahman) with different names and forms.