Question

In what circumstances can a vice president becomes president

Answer 1

when president suddenly dies.
when he resigns without notice.
when he is found to be corrupt.
and there is an empty seat of president due to above causes

Answer 2

In the case that the president can no longer serve, the vice president would serve as president. If the vice president cannot serve, the line of succession falls to the speaker of the House, then to the Senate president pro tempore, then to Cabinet members.