Sahasranamam was graced by Sri Bhismar, since he has coined the name Vishnor Nama Sahasram, we call it Vishnu Sahasranamam.
Notes:
Based on Vedas, the name Vishnu is an exalted name of Perumal. The first half (pUrva bhAgam) of Veda while talking about Yaga-s predominantly mentions the name Vishnu from among the divine names of Perumal. This is also the most celebrated name in Veda, we also follow the same.
It is not wrong to chant other hallowed names of Perumal as Andal eulogized “Namam Ayirametha Ninra Narayana”.