How about you use an actual image of a bubble, mask it with a spherical mask and compose it (I used screen) over a video thats also masked out and uses (optional) a spherize effect? Than precompose the whole thing and you can reposition the whole bubble with its content.
I used the track matte option to get the video into a cloud shape I made. However, the cloud is only showing a small section of my video. How do I scale the video layer and center it in the cloud shape? It won't let me scale the video. I know it's possible because it automatically does it when I choose the sphere effect.