It depends on what tactic you want to take with the paper. - Persuade the reader why Vishnu is a good religious entity, to the Hindus, or anything like that.
- Persuade the reader why Vishnu has good values in mythology and religion.
- Persuade the reader what makes Vishnu a good character in philosophy/religion/mythology via his avatars and other qualities.
I'm sorry if my ideas sound basic, but it's been a long time since I've read about Vishnu, and I can't really think up a good topic at the moment. Chances are you've taken notes on brainstorming. While these tips that the English courses provide are stupid, they are a good idea when thinking up your thesis statement. To figure out which direction you want to go with your paper, come up with different arguments, and figure out which one is the best to prove/argue, for a persuasive paper.