Here both played a significance role
In samudra mantana mandara parvata used to churn ksheera saagara (which belongs to God Perumal),Perumal or vishnu being turtle supported the parvata.Vasuki is used as rope for mandara.All the gods (except perumal who was holding the mandaram on his back as a turtle) including bhrama and lord Shiva and on the other side all the demons.
God Perumal : supported entire programme as a kurmam and become basic energy probiding boath to gods and demons
Lord shiva : drank the poison kaalakuta which has a power to destroy entire universe. After drinking it starts affecting shiva by creating a unbearable burning sensation in his throat looking lord’s condition parvati look after the kaalakottam not to enter his throat. So she did not allow him to sleep and this is celebrated as shivaratri jagaranam