At the recent Sundance Film Festival, virtual reality was a big hit for the second year running. But the changes in just a year were also evident: rather than the conversation focusing on the technology enabling the VR experiences, the focus was on storytelling. Virtual reality, in other words, has become a medium, as demonstrated in projects such as “Grease” director Randal Kleiser’s sci-fi VR drama “Defrost,” which tells the story of a woman just awaking from a coma. The series is a full season with another in development.