Was Rāmānuja the first author of the Vedāntisation of the current(s) which later became well-known as Viśiṣṭādvaita Vedānta? Possibly yes. But, one might suggest that there are many Upaniṣadic quotations also in Yāmuna’s Ātmasiddhi and that Rāmānuja’s Śrībhāṣya seems to speak to an already well-established audience, and I wonder how could this have been the case if he were the first one attempting the Vedāntisation…
Carman (2007, pp. 63–64) suggests a different solution, one in which the Vedāntisation (my terminology) had already been undertaken by Yāmuna in the case of Pāñcarātra (which is, instead, rather neglected in Rāmānuja’s three theological works):
Yāmuna’s efforts to incorporate Pāñcarātra doctrine into the Vedānta involve some recognition of the Bhedābheda view permeating Pāñcarātra. In order to avoid Śaṅkara’s criticism of the Bhedābheda, however, Yāmuna has to develp his own distinction between brahman in the pure state and brahman as the creative power behind and within the universe. For him, this distinction is between God as the possessor of qualities and the divine qualities thus possessed, out of which the universe evolves. He admits* that his view can be described as “difference and non-difference”. […] Rāmānuja appears to be more precise than Yāmuna and more consistent in avoiding expressions that sound either like Bhāskara’s Bhedābheda or Śaṅkara’s Advaita. Perhaps this was an additional reason for Rāmānuja […] to avoid turning to Pāñcarātra texts.
To sum up, according to Carman (1974 and 2007) and Neevel (1977), Yāmuna already initiated a Vedāntisation. The main differences between his Vedāntisation and Rāmānuja’s one would be the fact that the former focused on the Vedāntisation of Pāñcarātra and was closer to Bhedābheda.
Rāmānuja, I might further suggest, was creating a Vedāntic school much more than he was vedāntising anything else. Furthermore, he appears to have been the inventor of the Viśiṣṭādvaita ontology.
*Unfortunately, Carman does not say where. Since he refers to Neevel 1977 in this paragraph, one might try to look there.