I reprinted a portrait using the sharp layer and a diffused layer, blended togther. It looked good to me. The nice thing about this is, the results can be changed at will. If you diffuse the original, you areprety much stuck with it.
As for taking images, I like to use a diffusion filter like a Softar I, II or III. Expensive, but the results are predictable and repeatable. Breathing on the lens leads to varying results. Using vasoline on a skylight filter is messy at best. Dirt sticks to it and it gets all over me and the rest of my gear, then dirtsticks to everything.
I looked at the Canon info on their soft focus lenses. It appears the effect is different from defocusing the original. I have used the Mamiya soft focus 135mm lense and it is one of the sweetest portrait lenses I have ever seen. When used by a real expert (not me), it really produces nice images.