Reference to Lord Venkateshwara in Vedic Puranas
Maha Vishnu as Venkateshwara(Srinivasa) with names as Venkateshwara and Srinivasa is clearly mentioned in the Puranas like Varaha Purana, Padma Purana, Brahmanda Purana, Garuda Purana, Markandeya Purana, Brahmottara Purana, skanda Purana, Aditya UpaPurana etc. These Puranas has clearly mentioned the names Venkatesha and Srinivasa of Seshadri pati(Lord of Seshadri hills) and venkatachala vasa.
Sri Venkatesha Mahatmyan (Glories of Lord Srinivasa) is mentioned in the Varaha Purana and Aditya UpaPurana.
In Brahmanda Purana there is a Prayer (stotra) by Brahma deva to Lord Sri Venkateshwara. Brahmanda Purana also has Venkateshwara Sahasra Nama and Venkatesha Astottara Nama of Lord Sri Venkateshwara which is uttered every morning in Tirumala after first bell and another Astottara Nama from Varaha Purana after second bell.
In Brahmanda Purana mentions Vyshista Maharshi in Treta Yuga explained Venkateshwara and Tirumala Glories to Narada.
Lord Sri Venkateshwara's great posture on Tirumala is glorified in the Bhavishyottara Purana.
Markandeya purana mentions Sri Venkateshwara Vajra Kavacha Sthothram by sage Markandeya.
And also in Skanda Puranam there is prayer named Sri Venkatesa stotram.
Padma Purana mentions about Goddess Sri Padmavati Tayar and Srinivasa marriage.
The Mantras uttered in the Tirumala temple for Lord Sri Venkatesha is mentioned in the Brahmanda Purana and Varaha Purana.
SrI vEdavyAsa encapsulated, the proclaimed mahima, in another masterpiece, called SrI vEmkaTESa mAhAtmyam as part of AdityapurANAmtargata prabamdham and this was told to all Rushi's by Suta Maharshi who also told Srimad Bhagavatam.