I would like to read Ramanujacharya's commentary on the Vishnu Purana in English online. Can anyone provide me the relevant link?
Ramanujacharya didn't write any commentaries on the Puranas. Ramanujacharya only composed nine works: the Sri Bhashya, the Vedanta Dipa, and the Vedanta Sara, three commentaries on the Brahma Sutras; the Bhagavad Gita Bhashya, a commentary on the Bhagavad Gita; the Vedartha Sangraha, an independent exposition of Visistadvaita; the Sharanagati Gadyam, the Sri Ranga Gadyam, and the Vaikuntha Gadyam, three prose hymns on the subject of Sharanagati; and the Nityam, a work about the daily activities of Sri Vaishnavas. (All these works are available in English except the Nityam.) He didn't compose commentaries on any Puranas including the Vishnu Purana.
But other Sri Vaishnava Acharyas have composed commentaries on the Vishnu Purana (and the Bhagavatam also). Unfortunately none of them have been translated into English. But they're available in Sanskrit. The most famous one is the Vishnuchittiyam, composed by the Sri Vaishnava Acharya Engalalwan who was a shishya of Ramanujacharya's cousin (and disciple) Thirukurugai Piran Pillan. You can read it in Sanskrit here.