What is the best way to transition from 3D to VR in the same game?

At first I was all-about making a VR multiplayer game, however there are certain parts that would not work well in VR at all. What is the easiest way to build a game with 2D or 3D rooms that transition into VR? Is it even possible to do it seamlessly? Or are there apps and programs already dedicated to this that I haven't been able to find? Or would it be better to buy Unreal? 

Thanks in advance.