This article shows a composition example: https://software.intel.com/en-us/articles/video-composition-using-intel-media-sdk
To change composition position, you would need to change the settings in the composition filter extended parameter buffer. To do this you would need to "drain" any frames currently buffered by VPP then reset or close/init VPP to read in the new parameters.
This could be a reasonable approach if the position changes infrequently. If you want to change position for each frame you may be able to write your own optimized custom composition with OpenCL.