The legend of samudra manthan and amrita is mentioned in itihasa, puranas but the legend of samudra manthan mentioned in verses of vedas, Upanishads or brahmanas or is the drink of immortality amrita atleast mentioned in these Vedic scriptures that is vedas, Upanishads or brahmanas?
I could not find refernces of samudra manthan in vedas but I found refernces of amrita-
for,wealthy waters,ye control all treasures:ye bring auspicious intellect and amrta ye are the queens of independent riches sarasvati give full life to the singer!(Rig veda book 10 hymn 30 verse 12).
Now,ye sapient ones,make ye the axes sharp wherewith ye fashion bowls to hold the amrta.Knowing the secret places make ye ready that whereby the gods have gotten immortality(Rig Veda book 10 hymn 53 verse 10)