The official word from Oculus is that you should not have the SteamVR plugin enabled for any applications which you package for the Oculus Store. If you package for Steam, you can have the SteamVR and Oculus plugins enabled.
In my VR project, I haven’t really experienced notable framerate differences between the Vive and Rift platforms. If you are hitting 45 FPS, it means you’re getting reprojection, where the hardware switches from 90 FPS to 45 FPS by showing the same frame twice so that the game can catch up. That generally means that you’ve got some performance bottlenecks you need to identify and fix. Use the profiling tools within UE4 to figure out whether you’re CPU bound or GPU bound and then work to shave off those precious milliseconds. Framerate optimization for VR is almost a black art.
Edit Note: 90 / 2 = 45. The hardware refresh rates for the HTC Vive is hard fixed at 90 frames per second, and it will always render 90 frames per second. If your app can’t keep up, the hardware driver switches into reprojection mode. This is always, always an optimization problem if you are running on minspec hardware.