Bhakti movement, which is most prevalent in Vaishnava sect cannot be traced back to the Vedas. Rigvedic God Vishnu had initially nothing to do with this. This is a pre-Vedic non-Aryan cult which later got fused with Vedic religion. This is how it happened.
At the end of the Upanishadic period (8th to 6th centuries BCE), Vedism was in decline. It was a ...
You can read an English translation/commentary of an abridged (subset) version here.
The various schools refuted here are:
Prachchanna Bauddha (Nirvesesha/Sankaran Advaita)
Bhaskara / Yadava School
Vishnu Purana is accepted as Pramanam, logical argument, by Ramanuja and his lineage of scholars.
As such, we concur that the Supreme Being is of Form outside our level of understanding. However, Out of His Boundless Compassion, He appears as Five Forms such that He can communicate and interact with us.
As Vishnu, He Pervades and Operates this Universe ...
Here is the dialogue of Parāśara Muni in the website you have quoted (I have added Sanskrit transliterations wherever possible):
Parāśara Muni:— Having offered salutation to the mighty and indescribable Viṣṇu, I repeat to you what was formerly related to me by Vasiṣṭha. The glorious Hari wears the pure soul of the world, undefiled, and void of qualities, ...