The whole Vishnu Purana is encircled around Lord Vishnu ji and one thinks that worshiping him can give us happiness but it is not true. As mentioned in Shreemad Devi Bhagwat Maha Puran, Sachitra Mota type Hindi, Printer Hanuman Prasad Poddar- Chimanlal Goswami, printed by Gita Press Gorakhpur, Concise Devi Bhagwat page no 123, Skand 3, Adhyay 5 provides evidence that Brahma, Vishnu and Shiva are into the cycle of birth and death. Hence not immortal.