Switch between normal and split-screen (vr mode)
I don't think that is possible yet, but google is working on DayDream which is gonna do the thing you want soon 
If you want to play your normal videos in VR mode, thats possible using this app. (There are quite a lot other apps doing the same thing, but i personally use the one I gave link to)

Heres one more link for 5 Top Videos players to run 360 Videos 
Hope this helps :)