The Aadi Naam of Vishnu is OM.
The word विष्णु comes from the root विष्. The word Vishnu means ALL- PERVADING (व्यापकत्वात् विष्णुः).The word ब्रह्म comes from the root वृहि, which means expansion.The word Brahma means THE BIGGEST ONE (बृहत्वात् ब्रह्म). Only the biggest one can be all pervading. So Brahma and Visnu are synonymous. Vishnu can be Nirguna or Saguna according to mental propensity of the aspirant, as is mentioned in Srimadbhagabatam:
वदन्ति तत्तत्त्वविदस्तत्त्वं यज्ज्ञानमद्वयम्।ब्रह्मेति परमात्मेति भगवानिति शब्द्यते॥ which means that the realized souls name the same Unique One by words like Brahma, Aatma or Bhagavan.
OM is the Primal Name of Brahma or Paramatma, as stated in Srimadbhagavatam(12/6/41-42).
Actually OM is the source of all letters and therefore all names.
In Gita, Sri Krishna says:
ओमित्येकाक्षरं ब्रह्म व्याहरन्मामनुस्मरन्।
यः प्रयाति त्यजन्देहं स याति परमां गतिम्।।8.13।।
English Translation By Swami Gambirananda.
He who departs by leaving the body while uttering the single syllable, viz Om, which is Brahman, and thinking of Me, he attains the supreme Goal.
पिताऽहमस्य जगतो माता धाता पितामहः।
वेद्यं पवित्रमोंकार ऋक् साम यजुरेव च।।9.17।।
English Translation By Swami Gambirananda
Of this world I am the father, mother, ordainer, (and the), grand-father; I am the knowable, the sancitifier, the syllable Om as also Rk, Sama and Yajus.
रसोऽहमप्सु कौन्तेय प्रभास्मि शशिसूर्ययोः।
प्रणवः सर्ववेदेषु शब्दः खे पौरुषं नृषु।।7.8।
English Translation By Swami Gambirananda
O son of Kunti, I am the taste of water, I am the effulgence of the moon and the sun; (the letter) Om in all the Vedas, the sound in space, and manhood in men.
I must mention here that OM is NOT a syallable. The word Akhshara means unperishable and not syllable here.
The Vishnushasransama starts with OM.
So the conclusion is that the Primal Name of Vishnu is OM.
