The trinity of Lord Brahma, Vishnu and Mahesh is what the entire Hindu religion is based on. It is believed that Brahma is the creator, He is the administrator and Mahesh is the destroyer.
He has saved the universe with his ten avatars of which the last two are Lord Ram and Lord Krishna.
Many Vedas suggests that Lord Vishnu runs the entire universe and whenever he feels that the world is not balanced enough, and is filled with evil and demons, he takes an avatar and destroys the evil to regain the balance.
There are many Vishnu Mantras used for different benefits, few of them are listed below. By chanting these you can please him and fulfill all your desires.
ॐ लक्ष्मीपति विष्नाय नमः ।
Om Lakshmipati Vishnaya Namah
Lord Vishnu is husband of Goddess Lakshmi (god of wealth).
This Mantra will help you impress God Vishnu who in turn will shower you with loads of wealth and prosperity.
ॐ विष्णू श्री श्री श्री काम बीजाय फटू ॥
Om Vishnu Shri Shri Shri Kaam Bijaaya Fatu
Every time you start a new work or project, you pray to God for its success. The above Vishnu mantra is a powerful Beej mantra (from a very famous guru).
This mantra will help you get blessed by Lord Vishnu who in turn will grant you success in all your work.
ॐ नमो परर्मात्मने पार विष्णु मम शरीरे पाहि पाहि कुरु कुरु स्वाहा॥
Om Namo Parmatmane Paar Vishnu Mama Sharira Pahi Pahi Kuru Kuru Swaha
This is for good health. If you want your life disease free then chanting it will be useful.