The Viśiṣṭādvaita Vedānta is a philosophical and theological school active chiefly in South India, from the last centuries of the first millennium until today and holding that the Ultimate is a personal God who is the only existing entity and of whom everything else (from matter to human and other living beings) is a characteristic. As its name already says, Viśiṣṭādvaita Vedānta sees itself as a Vedānta school, i.e., as a school recognising the Vedāntasūtras (also called Brahmasūtras) as one of its foundational texts.
The beginning of Viśiṣṭādvaita Vedānta as an independent school are usually connected to Rāmānuja (traditional dates 1017–1137), both in India and in Western scholarship (e.g., in the work of the eminent Viśiṣtādvaita Vedānta scholar and former director of the IKGA institute Gerhard Oberhammer). Rāmānuja was indeed the first one to write a new commentary on the Brahmasūtra, thus robustly collocating his school within Vedānta. Again both Indian and Western scholars agree also on the pivotal role of Veṅkaṭanātha (also known with the honorific title of Vedānta Deśika ‘teacher of Vedānta’, traditional dates 1269–1370). My research (see, e.g., Freschi’s Veṅkaṭanātha/Vedānta Deśika, forthcoming on IEPh) suggests that this role is even more important than it has been recognised so far. Veṅkaṭanātha’s systematising efforts have not just re-shaped Viśiṣṭādvaita Vedānta but, have in fact, shaped it into the school we are familiar with now. It was Veṅkaṭanātha, for instance, to include the heritage of the Tamil saint poets, the Āḻvārs, within Viśiṣṭādvaita Vedānta, and to recognise as authentic the works on devotion attributed to Rāmānuja. It is indeed true that traces of each of these tendencies can be detected also before Veṅkaṭanātha, but the very idea of looking for such faint traces testifies of how the concept of what should belong to Viśiṣṭādvaita Vedānta has been influenced by Veṅkaṭanātha’s own philosophical synthesis. In other words, it is because of this synthesis, that both devotees and scholars agree about the importance of devotion, of the Āḷvārs’ sentimental expression of a self-oblivious love for God, of Pāñcarātra ritualism, of the Pūrva Mīmāṃsā hermeneutical tools etc., within Viśiṣṭādvaita Vedānta.
Thus, any appreciation of Viśiṣṭādvaita Vedānta must focus on the impact of Veṅkaṭanātha, since it is only through such an analysis that one will be able to recognise innovations and continuities in “Viśiṣṭādvaita Vedānta” before and after him.
Veṅkaṭanātha was a very prolific author, to whom more than one hundred works in three different languages have been attributed (no reason for scepticism concerning this attribution is known to me). As for his philosophical works, Srinivasa Chari has started some decades ago a series of monographs dedicated to a paraphrase and analysis of many of them. More recently, a monograph on Veṅkaṭanātha’s concept of time by another researcher working at the IKGA, namely Marcus Schmücker, will soon be released. Nothing at all, by contrast, has been dedicated to Veṅkaṭanātha’s confrontation and eventual absorption of the much more ancient and influential school of Pūrva Mīmāṃsā within Viśiṣṭādvaita Vedānta, although the link to Pūrva Mīmāṃsā can be read (see Freschi’s Śrī Vaiṣṇavism: The making of a theology) as the key factor distinguishing Viśiṣṭādvaita Vedānta from the other concurrent school within Vedānta, namely Advaita Vedānta. Against Advaita Vedānta, Viśiṣṭādvaita Vedānta and Pūrva Mīmāṃsā share key tenets, such as the belief in personal souls, in the relevance of following Vedic prescriptions even once one has surrendered to God, etc.
(this post is meant to be a general introduction to the topic, accessible to non-initiated readers. Should you find something in it not understandable, please let me know with a comment below.)