There are many programs that you can use, but in this tutorial, I'm going to use Camtasia 7. It will convert my GIF format into WMV (Windows media Video) format. But you can use other format such as MOV (Quick Time) or MP4.
When inserting images into the track, the smallest duration Camtasia allowed for images is 0.1 second. So, if you rendered your Vray images using 15fps (frame per second) then you must edit the result again in camtasia and raise the clip speed value to 150% (but in this tutorial I use only 120% since it's smooth enough for me). Or if you use 30fps when rendering the animation, you probably have to raise clip speed up to 300%