Vishnu and Krishna
"...and wherever the Lord is present, the goddess of fortune is also there because the goddess of fortune never lives alone without her husband. Therefore, victory and fortune were awaiting Arjuna, as indicated by the transcendental sound produced by the conch shell of Vishnu, or Lord Krishna." (Bg. 1.14)
"...Persons who are led by the material conception of life do not know that the aim of life is realization of the Absolute Truth, Vishnu, or Bhagavan Sri Krishna, and they are captivated by the external features of the material world, and therefore they do not know what liberation is. .."(Bg. 2.2)
Simultaneously, Srila Prabhupada would speak of Lord Vishnu as that expansion of the Lord Who would take care of business in a more formal way - protecting His devotees, riding on Garuda (Lord Vishnu's great winged carrier), in the heart of each soul - and Lord Krishna as the original form of the Lord who appears in Goloka Vrindavan, plays with the gopas (cowherd boys), and has loving relationships with His dear devotees.
As in the quotes below, from the 9th Chapter of Bhagavad Gita As It Is:
"The mahatma (great soul) does not divert his attention to anything outside Krishna, because he knows perfectly well that Krishna is the original Supreme Person, the cause of all causes. There is no doubt about it. Such a mahatma, or great soul, develops through association with other mahatmas, pure devotees. Pure devotees are not even attracted by Krishna's other features, such as the four-armed Maha-Vishnu. They are simply attracted by the two-armed form of Krishna. They are not attracted to other features of Krishna, nor are they concerned with any form of a demigod or of a human being. They meditate only upon Krishna in Krishna consciousness. They are always engaged in the unswerving service of the Lord in Krishna consciousness." (Bg. 9.13)
. . .or, from the 11th Chapter of Bhagavad-gita:
"This chapter reveals Krishna as the cause of all causes. He is even the cause of the Maha-Vishnu, from whom the material universes emanate. Krishna is not an incarnation; He is the source of all incarnations. That has been completely explained in the last chapter." (Bg. 11.1)
It is summed up by Lord Brahma in his Brahma Samhita as follows:
jivanti loma-vila-ja jagad-anda-nathah
visnur mahan sa iha yasya kala-viseso
govindam adi-purusam tam aham bhajami
"The Maha-Vishnu, into whom all the innumerable universes enter and from whom they come forth again simply by His breathing process, is a plenary expansion of Krishna. Therefore I worship Govinda, Krishna, the cause of all causes." Therefore one should conclusively worship the personal form of Krishna as the Supreme Personality of Godhead who has eternal bliss and knowledge. He is the source of all forms of Vishnu, He is the source of all forms of incarnation, and He is the original Supreme Personality, as confirmed in Bhagavad-gita."(Brahma-samhita 5.48)
Also in Brahma Samhita Lord Krishna is described as the "original candle" from which many candles are lit, and they appear the same, but the original is Lord Sri Krishna!