It's very easy to produce mush with a soft focus lens, so I find the trick with soft lenses is to blend soft and hard. A portrait with a soft focus lens really uses the qualities of the lens to advantage with hard light. Where a contrast ratio of 1:2 or 1:3 is typical for portraits with lenses designed to be sharp, a diffuse focus lens calls for a contrast ratio of 1:4 or 1:5. A main light and no fill will often be just right, and when shooting with available light that isn't hard enough, it may be appropriate to extend development time to increase contrast.
I also find that I have a characteristic aperture. With a soft focus lens, you have to look at the whole groundglass and adjust focus and aperture until it looks right. You may use a loupe to make some particular detail sharp, but there is no "correct" focal point with a soft focus lens. If I'm using a Verito or Heliar (which isn't a soft focus lens per se) and I adjust the aperture by looking at the groundglass rather than the ring, I find I'm just about always around f:5.6. For whatever reason, that's where the balance of sharp and diffuse looks right to my eye.
Someone on the LF forum a few years ago posted a summary of an article in a photo magazine from the age of soft focus lenses where various professional portrait photographers were asked to focus the same image with a soft focus lens, and the focal points were marked on a strip of tape on the camera bed, and there was a wide spread among the various choices and no two photographers agreed on what was "in focus." Each photographer was painting with a different brush.