Rigveda does not show the monistic (Ekadeiva - only one Godhead behind everything) concept. That seems to me to be a later adoption, most probably when knowledge about the abrahamic religions reached our brahmins in the ancient days through traders etc., from the present day Gulf countries, Turkey, etc., as also from the zoroastrians through the North-West land route across the Hindu kush. Once the idea reached us, the early Upanishads - specially Brihadaaranyaka, discussions on Brahman, the later Upanishads, etc., started and culminated in what is now vedanta. In order to suit our convenience some riks like "Ekam sad vipraah bahudhaa vadanti" etc., were extracted out of context and tomtomed to make an impression that the much later one-Reality concept was already in the earliest of the vedas, imo. But I will like to be corrected if my impression is wrong, based on scriptural internal evidence.