Lord vishnu
Vasudeva is Maha Vishnu as Vyukam. He protects universe and all living things.
Vibhavan is Avatars of Maha Vishnu. Lord Krishna is Avatar(Vibhavan)
Vishnu is usually shown with light blue skin and four arms. He holds a lotus, mace (gada), conch {shankha) and disc (chakra) in each of four hands. He has famously taken many guises. These are the 10 Avatars of Vishnu (in the order they appeared): Matsya (Fish), Kurma (Turtle), Varaha (Boar), Narasimha, (half lion, half man), Vamana (Dwarf / Short man), Parashurama (Fierce man], Rama ( Ideal man ), Balarama { Strong man }, Krishna (Cowherd Prince), Kalki { Man on the White Horse } not appeared as yet. Some say that he has already appeared in the form of Buddha, which is disputed even by Buddhists.
Though the focus of the school of thought of Vaishnavite philosophy was on different platforms like focusing on Narayana or Krishna, focus on Krishna only but on both Radha and Krishna etc.,sheer Bhakti was focused in South India in the 7th to 10th centuries in poems that the Alvars composed in Tamil to Vishnu. Drawing on earlier Tamil secular traditions of erotic poetry as well as royal traditions, bhakti poets applied to the god what would usually be said of an absent lover or of a king. Bhakti soon spread to North India, appearing most notably in the 10th-century Sanskrit text the Bhagavata-purana. South Indian texts show close parallel with the Sanskrit traditions of Krishna and his gopi companions, so ubiquitous in later North Indian text and imagery. Early writings in Dravidian culture such as Manimekalai and the Cilappatikaram present Krishna, his brother, and favourite female companions in the similar terms. Historians argue that the Sanskrit Bhagavata Purana is essentially a Sanskrit "translation" of the bhakti based works of the Tamil alvars. Devotion to southern Indian Mal (Tirumal or Perumal) may be an early form of Krishnaism, since Mal appears as a divine figure, largely like Krishna with some elements of Vishnu. The Alvars, whose names can be translated "sages" or "saints", were devotees of Mal. Their poems show a pronounced orientation to the Vaishnava, and often Krishna, side of Mal. But they do not make the distinction between Krishna and Vishnu on the basis of the concept of the Avatars. The Bhagavad Gita is an important scripture not only within Vaishnavism, but also to other traditions of Hinduism It is one of three important texts of the Vedanta school of Hindu philosophy, and has been central to all Vaishnavism sampradayas.The Bhagavad Gita text is a dialogue between Krishna and Arjuna, and presents Bhakti, Jnana and Karma yoga as alternate ways to spiritual liberation, with the choice left to the individual. The text discusses dharma, and its pursuit as duty without craving for fruits of one's actions, as a form of spiritual path to liberation. The text, succinctly summarizes the foundations of Vaishnava theology that the entire universe exists within Vishnu, and all aspects of life and living is not only a divine order but divinity itself. Bhakti, in Bhagavad Gita, is an act of sharing, and a deeply personal awareness of spirituality within and without.The Bhagavad Gita is a summary of the classical Upanishads and Vedic philosophy, and closely associated with the Bhagavata and related traditions of Vaishnavism. The text has been commented upon and integrated into diverse Vaishnava denominations, such as by the medieval era Madhvacharya's Dvaita Vedanta school and Ramanuja's Vishishtadvaita Vedanta school, as well as 20th century Vaishnava movements such as the Hare Krishna movement by Swami Prabhupada.Thanks to the concentrated focus on the medium of Bhakthi , lot of temples were formed in all parts of India like, Nathdwara, Rameshwaram, Guruvayur, Dwarka, Ayodhya, Mathura, Vrindavan, Varanasi, Vaishno Devi, Pandharpur, Dwaraka,Dakor, Mukthinath,Kathmandu, Tezpur, Udupi,Triplicane, Tirupati, Srirangam, Badrinath, Jagannath, Mayapur, Srivilliputtur, Sri Vaikundam, Thirukkoshtiyur, Thiruvananthapuram, Tiruvalla, , Annavaram, Badrachalam, Melkote, Srirangapatnam, Tirucherai, Kanchipuram,Thirukkoyilur, Oppiliappankoil, Srimushnam,Thirukkannamangai, Thiruvidanthai,Mahabalipuram, Sriperumbudur, Mannargudi, Kumbakonam, Mohur,Tiruppullani, Madurai, Alagar Koil and the list can go on and on, if you list the Major Indian pilgrimage and temple sites in Vaishnavism.It is a great success of the devotees who laid their faith on Bhakthi and Surrender.
Mantra
Om namo Narayanaya Namaha
Om Namo Bhagavathe Vasudevaya Namaha