Translation:We are looking for someone with a vivid imagination.
No, a rich imagination works in English too, and has perhaps a slightly different meaning. Vivid would be more about detail, kind of like the quality of an image or video, rich would apply more to the extent or boundaries of imagination, more the size of the imagination than its detail.
Hahah I've never heard of this, and would have to say its probably nonsensical - fertile refers to growing something, like a fertile womb or fertile land (fertility always requires something else, in this case 'seed' in both the literal and euphemistic sense). A fertile imagination would therefore mean something along the lines of, you tell them an idea and they are able to flesh it out (grow it?) into a fully fledged novel/plot/concept in their head? Very strange, cant quite see how it would ever be applicable but its an interesting idea lol