You would be better off to trellis the vines. trimming them will take away the leaves they use in photosynthesis.
Sweet potatoes can sprawl pretty far, they need their space. I would suggest the trellis. Anything can be used. I used those square tomato cages you can get at Home depot etc last year for such things. They fold out in a straight line and work pretty well. Or you could make one with stakes and twine/string. If they back up on a fence that will work as well.