When using the vice grip edge breaker, you are not looking for any real pressure. You should be able to do your break with one pass. Set the rollers so that they are the thickness of the material apart. Just put the parallel part of the rollers over the edge,then adjust so they roll easily. Then lock them off. Open and get them to the full depth, then close them. I found the best result was obtained by holding them in position, then using my other hand to roll the rollers slowly along the edge. You really do only need one pass to get the (very slight) edge break you are looking for. They are not a force forming roller, just a couple of shaped wheels that impart a nice shape to the edge. I had similar difficulties at first attempt, but once I worked them out, I swear by them, not at them.
Hope you can make sense of my long winded description. Good luck