Akshara

As part of basic instructions of Shiksha and Sanskrit grammar, it is explained that among the Word-entities, both, Aksara and Brahman stand out as especially important because both refer to a special form of ritual word. In the Brahmanas and the Upanishads both come to mean the Absolute.

Vedantic philosophy identified the aum syllable as alluded to in various concepts going back to the Rigveda, such as the concept of the "word" or "vāc" (e.g. RV 1.55.1) or inspiration (1.34.4, 8.36.7). "It is the recipient of power and the dignity of the sacred word in a condensated and intensified degree, and as the essence and embryo of speech receives, more than the word itself the signification of transcendent Brahman".[2] In the Rigveda itself, akṣára does occur, but it is used as a name of "water" (RV 1.34.4, 1.164.42).

The Manduka Upanishad partitions the symbol Aum in three different morae and adds a fourth mora-less part instructing that the mora-less part alone is ultimately real and not the other three representing "wakefulness", "dream" and the "sleep" states of consciousness. The mora-less part of Aum has correspondence with the fourth dimension of metaphysics, the Atman.[3] Madhavananda in his commentary on the Brahmopanishad belonging to the Atharvaveda, explains that vide Mundaka Upanishad I.7 and II.1-2 the term Aksara signifies Brahman in Its aspect of the manifesting principle who Pippalada says is the thread (Sutram) to be worn instead of the sacrificial thread on the body which should be discarded.[4]

According to the adherents of the Smrtis, the practitioners of the Bhakti yoga, Aksara means one who is present everywhere, denotes the name of Shiva and Vishnu, and also that of Brahman, literally it means imperishable, indestructible. And, because it is the term applied to Aum it is called the Aksara, the symbol of God who is the lord of all created things. It is a descriptive synonym of Brahman (Bhagavad Gita VIII.3) who is said to have arisen from Aksara (Bhagavad Gita III.15).[5]

With regard to Akshara being an epithet of Vishnu take a look at Vaikuntha

With regard to Vallabha’s view of Aum it is said that Aksara itself is imperishable and appears as souls endowed with Sat and Chit but not as Ananda.[6] For Vallabha, Ananda, which is the first manifestation of God, is the actualisation of the absolute identity and selfness, whereas the second manifestation of God is the Aksara, the impersonal ground from which all determinations arise because it is the substratum of all finite forms that pre-exist but issue forth from it which though by itself is the intermediate form that lacks plenitude.[7]