I also have a Viking Sapphire and the Viking free motion foot I use is neither of those pictured...it is clear and has an open rectangle. It attaches on the regular arm just like your A and B feet. I did order the one you have as well for echo quilting (the guides are to keep you uniformly at a distance from the previous outline quilting) and have not used it as that was a recent purchase. My regular FM foot works well, but I think the one you ordered will do fine too.