I had issues like you had of crashing unity a lot of times. I believe its with the 64-bit version of the Unity which causing it. However, I did managed to successfully run the videoplayback in the 64-bit with struggling a lot with the crashes and errors in unity-64bit. But if you're doing a good amount of R&D I suggest to try unity32-bit only. A lot of times the video won't play, actually It never did for me, so, Yeah, You might wanna put in a IOS or Android device to check for your application with the whole Augmented Reality Videoplayback thing. Here what I will do, If I were you :
1. I will use reinstall 32bit unity 5.4 or above specifically, because I will be using Vuforia 6 which is the latest version, and so as to cope with versions and with the bugs and issues which I don't want to face again.
2. I will probably check with a ready made tutorial to see if it's me, missing some important part such as Licensing my app, properly using target, and have I activated the data sets properly or not. And also just simply try to build a ready made tutorial so as to check my software and plugins whether they are running perfectly or not.
*You might wanna checkout this man's work he used vuforia in a very efficient way with a good tutorial as well : https://www.ourtechart.com/augmented-reality/tutorial/augmented-reality-video-playback/
(Try to use the latest versions to avoid crashes and bugs)
*Here's a ready made tutorial to check your software's and plugins are running perfectly fine or not : https://docs.google.com/uc?authuser=0&id=0BygvzTqnzm_wd01NNUp4eldhMGs&export=download