I've done a couple of decal-to-mask scans, and I've used Inkscape (free) to do all of the scaling, grouping, line correction, dust removal, and cleanup. Inkscape will do a lot, but it isn't the easiest software to learn. You will waste some vinyl learning how to use it but it can give good results.
I have access to a Cricut instead of a Silhouette, and I do not like the software included for any editing.
Michael