I don't know where is Lord Vishnu's Vaikuntha sea. Is it on Earth or not?
Loka is a Sanskrit word for 'world'. It was not on earth as earth is called 'Mrityu Loka' and 'Vaikuntha' is a different Loka. One can only reach Vaikuntha Loka by doing bhakti of Lord Vishnu.
Vaikuntha means a place where there's no kuntha (worries/suffering). So definitely this planet can't have vaikuntha.
As far as locating the same is concerned, best of luck but unless you've your kundalini awakened and raised, you can't locate or experience Vaikuntha.
As per the scriptures, in Srimad Devi Bhagavatam:
So there are endless Brahmâs, Visnus, and Mahes’varas. In every Brahmânda, there is Brahmâ, Visnu, and Mahes’a. Each Brahmânda extends from Pâtâla to the Brahmâloka. The abode of Vaikuntha is higher than that (i. e. it is situated outside of Brahmânda), again the abode of Goloka is fifty koti yojanas (50 x 10 x 4 x 2 million miles) higher than Vaikuntha. This Goloka Dhâma is eternal and real as S’ri Krisna is eternal and real.
vaikuntha is heaven where everyone there is eternal, there is no old age, no disease and no birth death and no rebirth. Lord Vishnu is eternal,no change and has no form and no name. he has 1000 names . they worship his incarnations and him the god of love and peace. everyone sings and chant his glory with bliss and love. and they stay with Lord Vishnu forever and they never have to fall down to the misery material world again to suffer. and Lord Vishnu stays and lives in vaikuntha forever. and he loves people who read stories about him and loves hearing the stories. to reach Vaikuntha is to be good and pure clean soul,improve yourself and do good deeds , put other people first and yourself last and chant and meditate on him and Lord Vishnu will take u to Vaikuntha by a flying eagle bird named Garuda or one of his messengers will pick u up on a flying swan ship and take u straight to Vaikuntha.