The answer to the same is both a Yes and no. Lord Vishnu and Shiva are the same as they are the core energy force and have originated from the same energy source. They are the same as they are manifested by the same energy.
It is believed that the Trinity - Brahma, Vishnu and Mahesh often give rise to each other and are a single entity with different purposes.
On the other hand, Shiva and Vishnu are a form of each other. One cannot exist without the other as their powers of protection and destruction go hand in hand.