It seems that Vaishnavism isn't as popular as I had initially expected it to be. What groups venerate Vishnu other than ISKCon? Is there a particular region of India were Vaishnavism is the majority? If not, where is it most popular?
First of all, according to legend, Sri Hari is said to have taught His worship to His four devotees: Lakshmi, Shiva, the Sanat-Kumaras & Brahma. They, in turn, founded the four main Sampradayas (sects) respectively: Sri Sampradaya, Rudra Sampradaya, Kumara Sampradaya & Brahma Sampradaya.
Each of this Sampradayas have further divisions.
The Sri Sampradaya founded by Goddess Lakshmi which subscribes to the philosophy of Vishishtadvaita, has three main divisions:
Sri Vaishnava Sampradaya: The adherents of this sect are mainly found in the states of Tamil Nadu & Andhra Pradesh in South India. Founded by the great Vaishnava Nathamuni (823CE-924CE), this Sampradaya stresses on Sharanagati to Sriman Narayana and His avataras. The Naalayira Divya Prabhandham, the Pancharatra Agamas and the Vishnu Purana are considered the main authorative texts in this Sampradaya. The sect has further subdivisions: Thenkalais and Vadakalais (the differences between the two are discussed here).
Swaminarayan Sampradaya: Mainly based in Gujarat, this Sampradaya was founded by the great Vaishnava, Sahajanand Swami (1781-1830). The main texts in this Sampradaya are the Vachanamrut and Shiksapatri. The aspect that makes this Sampradaya different from other Vaishnava Sampradayas is that it believes in non-difference between Vishnu & Shiva.
Ramanandi Sampradaya: Founded by the great Vaishnava Ramananda (1400-1476), devotees of this Sampradaya are mainly based in Northern India, especially in the state of Uttar Pradesh. The sect revolves mainly around the worship of Lord Rama who they consider the source of all incarnations as well as of Vishnu. The Ramcharitmanas & the Valmiki Ramayana are the main texts of this sect. The famous saints, Tulsidas and Kabir belonged to this sect.
The Rudra Sampradaya founded by Lord Shiva, which historically had multiple divisions, is now almost limited to the Pushtimarga sect which is mainly based in Rajasthan and Uttar Pradesh (founded by Sripad Vallabhacharya). Based on the philosophy of Shuddhadvaita, the main texts in this sect are the Shodhash Granthas and the Srimad Bhagavatam. Krishna is considered here to be source of all incarnations as well as of Vishnu. The sect revolves mainly around the worship of the Srinathji form of Shree Krishna.
The Kumara Sampradaya is the only Vaishnava Sampradaya which both historically and in the present, had/has no divisions. Their main text is the Srimad Bhagavatam and they too consider Krishna to the source of all incarnations as well as of Vishnu. Vaishnavas belonging to this sect subscribe to the philosophy of Dvaitadvaita.
The Brahma Sampradaya founded by Lord Brahma, has two main divisions:
Madhwas: Followers of the Dvaita philosophy of Madhvacharya (1238 -1317) belong to this sect. This sect is mainly based in the Southern India, especially in the states of Maharashtra and Karnataka.
Gaudiya Vaishnava Sampradaya: This is the Sampradaya which ISKCON belongs to. Mainly based in Eastern India, this sect mainly revolves around thee worship of Radha-Krishna who they consider the source of all incarnations as well as of Vishnu. The sect subscribes to the philosophy of Achintya-Bheda-Abheda. Like most Krishna-centric Vaishnava traditions, the Srimad Bhagavatam is considered the main text along with the Brahmavaivarta Purana and the Chaitanya Charitamrita.
Outside these four Vaishnava Sampradayas, there are also other Vaishnava traditions such as
Vaikhanasas: Mainly based in South India, especially in the state of Andhra Pradesh, Vaikhanasas are mainly Brahmin devotees who stress on the importance of rituals. Vishnu is considered the supreme here and the source of all incarnations. The main texts in this sect are the Vaikhanasa Agamas and they subscribe to the philosophy of Lakshmi-Vishistadvaita.
Varkaris: Based mainly in the state of Maharashtra, the Varkari traditon revolves mainly around the worship of the Vithoba form of Vishnu. Founded by Jnaneshwar, the tradition subscribes to the philosophy of Advaita Vedanta. The Haripath & the Amrutanubhav are considered the main texts of this sect.
Assamese Vaishnavas: Regardless to say, Vaishnavas of this sect are mainly found in the state of Assam. Mainly revolving around the worship of Krishna, this Sampradaya subscribes to the philosophy of Advaita Vedanta. The main texts in this sect are the Srimad Bhagavatam, Kirtan Ghoxa and the Naam Ghoxa. Founded in the fifteenth century by Srimanta Sankardeva (1449-1568), the Sampradaya stresses on chanting of holy names.