The Main Mantra for Vishnu
Om Namo Narayanaya
Om Namo Bhagavathe Vasudevaya
Namo = Salutation
Narayana = Other name for Vishnu, Lord who resides in all the creatures
Bhagavan = The indeed, Lord, the name of God or a Avatar. Literally means possessing fortune, blessed, prosperous. This means, that he holds the six divine qualities
Vasudevaya = Krishna
It is a mantra said in times of trouble to re-establish harmony and balance