We at Vungle pride ourselves in our user experience. We have purposely given our developers the flexibility to set the orientation of the video. That said, we highly recommend the auto-rotate option, so that the video orientation can match how the device is being held. For portrait apps specifically, auto-rotation enables the user to rotate the phone so that the video can take over the entire screen and potentially lead to higher conversions.
There are several possible reasons that your videos are not auto-rotating, because there are quite a few ways to set orientation within an app. The most common reason is that orientation was set at a project level. Even though Vungle's default setting is to autorotate, a master orientation setting like that would cause us to be in portrait too.