In Sanskrit grammatical and philosophical discourse, vācaka refers to the word or expression that carries denotative power — the signifier that points toward a referent (vācya). This relationship between signifier and signified is of fundamental importance in the Viśiṣṭādvaita analysis of the Tirumantram.
Piḷḷai Lokācārya explains in Mumukṣuppadi that the praṇava 'Oṃ' is the vācaka of Nārāyaṇa. This is not merely a grammatical observation: it establishes that the very first syllable uttered in the Tirumantram is a direct, unmediated pointer to Bhagavān Himself. The praṇava encompasses the entire triad — the denoting word, the relationship of denotation, and the denoted reality — and the philosophical unpacking of this syllable reveals the nature of the jīva's complete dependence on the Lord.
The vācaka-vācya relationship also underscores the intimacy between language and reality in the Vedic tradition. The Āchāryas hold that the names of Bhagavān are not mere conventional labels but are inherently connected to His nature. Thus to utter the vācaka with understanding is to invoke the vācya — the Lord Himself — and this is why the praṇava occupies the supreme place among all sacred utterances.