The description of Vishnu is not found in the Vedas .
Narayana Suktham speaks of Narayana as the Principle, it may be noted that Narayana is different from Vishnu.
Vishnu Suktham again describes the principle of Vishnu.
Sketchy description of Vishnu in Human form is found in the Puranas, especially in the Vishnu Purana,
Dhuruva Stuthi describes Vishnu’s Form, but does not provide details about His Ornaments and marks .
Gopala Uttara Tapini Upanishad( 75-78) has the first reference to Lord Vishnu’s Form.
‘His feet bear the auspicious signs of a celestial standard, a royal parasol.
His chest is adorned by srivatsa locks of hair, the brilliantly shining kaustubha gem and rows of forest-flower garlands (vanamala).
His four hands hold shankha (conch), chakra (discus), gada (mace) and padma (lotus).
He is adorned with armlets, garlands, jewels, diadem and earrings shaped like makara the sea monster (makara-kundala )
His form is enchanting and auspicious (divya mangala vigraha); radiant like the sharad -full moon ; his eyes glow like blue lotus blossoming amidst the
pool of clear water ; his brows like a well strung bow; his nose slender and shapely like the petals of champak flower ; the serene, cool, gentle smile, pure
like cow’s milk , dancing on his full and well shaped red lips lights up the whole world ; his chin firm and well proportioned; his throat bright and sound
shaped like a conch; the tilaka adorning his forehead between the brows is luminous like crescent moon in a clear cloudless sky; his arms long , strong and
supple like the elephant’s trunk; his chest wide , strong and healthy; he is adorned with golden-hue–silk garments (pitambara);he is richly and tastefully
ornamented; and he is the very embodiment of all the grace , beauty and joy in the universe.
How Sri Vathsam came into Being.
Once the foot of Trivikrama came to the place where Bharadwaaja Maharishi was performing Japam, he struck on the chest of VishNu with a handful of water. The spot where he was hit became the mole and this mole is called Srivatsam( Mahaabhaaratham Saanthi Parvam 3.5.2 vide Page 401 of Notes on BalakaaNDam of Vaalmiki Ramayanam by Sri C.R. Srinivasa Iyengar).
There is another interpretation found in the same Santhi Parva 3.5.2. According to this, on one of the several occasions in which Lord Siva came to fight with Mahavishnu, he aimed his trident (Soolam) at the latter creating a scar on Vishnu’s chest and this scar is Srivatsam.
There is also the view that Lakshmi resdies in the Sri Vathsam.
We find a near complete description of Lord Vishnu in the Vishnu Sahasranama.
Anivarti nivrutatma samkshepta kshema-kruchivah
Shrivasta-vakshah shrivasah shripatih shrimatam-varah ..64 Vishnu Sahasranama.
sasanka chakram sakireeta kundalam
sapeetha vastram sarasee ruhekshanam
sahara vaksha sthalashobhi kausthubham
namami vishnum sirasa chathur bhujam
Megha Shyamam Peetha Kausheya Vcham
Shree Vatsangam Kausthubho Bhasithangam
Punyopetham Pundari Kayadaksham Vishnum Vande Sarva Lokaika Natham- Dhyana Slokas Vishnu Sahsranama