as a concept included in the various Vedanta traditions, what specifically does Ramanuja say about this concept? does he preach to worship only Vishnu avataras? Or does he allow a model closer to Smarta like Shankaracharya?
Not sure what you mean by a model closer to the Smarta. All the schools of Vyasa, all modern day Vedantists, assert Iswara. Sri Ramanuja's commentary on verse 2.1.3 of his Brahma Sutras Sri-Bhasya, he writes (Swami Vireshwarananda translator):
3. By this the Yoga philosophy is (also) rejected.
...This view is refuted by this Sutra. Even Hiranyagarbha is an embodied being and as such liable to error. This Smriti [of Kapila] also, like the Samkhya Smriti, is based on error. If Iswara is merely the efficient cause, then by knowlege of one everything will not be known. For the knowledge of the Pradhana [by Kapila] will not give us the knowledge of the sentient souls as they are not products of Iswara who, according to Yoga philosophy, is merely the efficient cause and not the material cause also. So we have to conclude that Brahman is alone both the efficient and material cause of the world and its knowledge will result in the knowledge of everything. Therefore, only those Smritis are authoritative which teach Brahman as the cause of the world. It may be said that the Yoga Smriti teaches many things which are also taught by the Vedas. It is true, and to that extent only is it acceptable, and its other teachings have to be rejected.