I just finished up a tutorial on how to do live green screen keying using vMix 16:
Hope you find it helpful! Also, there is a tutorial below that one that goes into a work flow for weather graphics too.
My next tutorial will go into using an HD file with the virtual studio input which allows you to do live on air camera moves as well as a few other features. Hope you enjoy them...if you have any questions, just let me know.