I don't think people will use .vdb to export complete cloudscape from Terragen to replace a sky backplate. (Files generated will be way too huge).
But a way to export localized cloud would be awesome. (If it's technically possible).
Mostly all render engine now can render .vdb volumetric, maybe not as good as Terragen, but it's possible to get really nice result.
But here are not many tools to generate 'realistic' clouds shape, and the existing one are really slow, because they work directly with voxel grid.
Changing a setting in an houdini cloud rig can take many minutes just to re-generate the grid.