For FMQ, you need a "hopping" foot. This is a foot that "hops" between stitches to allow you to move the fabric. A darning foot is a hopping foot. This type of foot typically has a spring that allows the hopping.
The left foot in your photo is an embroidery foot and does not hop. I'm not sure what the other foot is, but it doesn't look like a hopping foot.
A walking foot can be used to quilt straight lines, or even wavy lines, but it cannot be used for free-motion quilting.
It looks like your Viking 6430 is a low-shank machine. This means that any generic low-shank darning foot should work. I think any JoAnn Fabric that has a sewing machine section would likely have this foot. Otherwise, you can go to
http://www.sew4less.com and do a search for presser feet for the Viking 6430, and that will show you pages and pages of generic presser feet (including a darning foot buried in there somewhere).