The tradition calls itself ubhaya vedanta, the Vedanta of both, and the claim is deliberate and unusual. Alongside the Sanskrit Vedas and Upanishads it takes as revelation the Tamil hymns of the Alvars, twelve poet saints of the sixth to ninth centuries whose verses are songs of longing addressed to Vishnu in the temples of the Tamil country. One of the twelve, Andal, is a woman, and her poems are sung at weddings and in temples to this day.
The joining was institutional before it was philosophical. Nathamuni, in the tenth century, is credited with recovering the scattered Tamil hymns, arranging them as a collection of four thousand verses, and setting them to be sung in temple worship, which is what made a vernacular corpus into liturgy. His grandson Yamuna developed the theology, and Ramanuja, in the eleventh and twelfth centuries, gave it its finished form and reorganised the temple administration at Srirangam.
The consequence is a tradition in which a Sanskrit commentary on the Brahma Sutras and a Tamil devotional poem stand in the same category of authority, and in which temple worship, not private meditation, is where religion mainly happens. It is also why the tradition has a lineage of teachers, an acharya succession, rather than an order of renouncers alone, and why so much of its literature is commentary on the Tamil hymns written in a mixed Sanskrit and Tamil register.