Lord Krishna and Lord Vishnu are the one and the same. There is absolutely no difference in Tatwa. Lord Krishna resides in Goloka and he in his Vishnu form (Narayan) resides in Vaikuntha.

Lord Krishna was worshipped before Dvapar too. In the Srimad Bhagvat Puran which is the best among all the Puranas as confirmed by Gadud Puran and the Srimad Bhagvatam itself and Commented by several great Acharyas, it's the greatest Granth. And in Srimad Bhagvatam there are Referrence that prove that people worshipped Lord Krishna even before his Advent on Earth in Dvapar.

These are the few examples

In the Story of Ambarish Maharaj(It happened in Satyug) this is there

SB 9.4.18-20: He was sure to fix his mind upon the lotus feet of Kṛṣṇa, to use his words to describe the qualities of [the Lord of] Vaikunṭha, to use his hands for matters as cleaning the Lord's temple and to engage his ears in listening to the transcendental talks about the Infallible One. He used his eyes to look at the deities, the temples and buildings of Mukunda, he used his body to be in touch with the bodies of the devotees, he used his nose to smell the fragrance of the tulsī leaves on the lotus flower formed by His feet and used his tongue to relish the food that is offered to Him. By using his legs to walk to the Lord's holy places, by using his head to bow down to the feet of Hṛṣīkeśa and by engaging his senses more in being a servant of Him than a servant of his lusty desires, he was alike the ones who seek their refuge in being attached to the Lord Glorified in the Scriptures [like Prahlāda e.g.].

In the Story of Ajamil which again happened before Dvapar it is mentioned

SB 6.3.29:(Yama to Yamadutas in Ajamila episode) They who run from the truth and fail in their duties unto Lord Viṣṇu, they whose tongues never express the names and qualities of the Supreme Lord, who do not carry Him in their heart or remember His lotus feet and not even once bowed their heads to Kṛṣṇa, bring them all before me.

SB 6.3.33: He who holds on to the honey of Kṛṣṇa's lotus feet, will not again desire to enjoy the illusory quality of nature [māyā] that brings distress. Anyone else however, who enchanted by lust tries to do something to cleanse the passion out of his soul, will surely find the passion reappear.

In the story of Indra killing the demon Vritasur (Happened in Treta Yug) it is mentioned where vishwarup tells Indra about some prayers

SB 6.8.20: May Keśava protect me with His club during the hours after sunrise, may Govinda holding His flute protect me early in the morning, may Nārāyaṇa, the Lord of all potencies, protect me late in the morning and may Lord Viṣṇu, the ruler with the disc in His hand, protect me during the hours at noon.

Even Prahlad Maharaj meditated on Lord Krishna as it's there in both Srimad Bhagvatam and the Vishnu Puran

nyasta-krīḍanako bālo jaḍavat tan-manastayā kṛṣṇa-graha-gṛhītātmā na veda jagad īdṛśam

SB 7.4.37: As a small boy he, in his full absorption in the attraction of Krishna, refrained from playing and seemed to be absentminded, having no understanding for worldly matters.