This can provide you some info straight to the point
Google says:
When you send an image to Vision API, we must store that image for a short period of time in order to perform the analysis and return the results to you. For asynchronous offline batch operations, the stored image is typically deleted right after the processing is done, with a failsafe Time to live (TTL) of a few hours. For online (immediate response) operations, the image data is processed in memory and not persisted to disk
Get more info on Google Vision FAQ about this question