MapSource is a trademark of Garmin, and almost all MapSource programs (if not all) will work on your Vista C, though some capbilities of some of them may not be available. You can check their published compatibility list at:
http://www.garmin.com/cartography/#
However most older maps that aren't listed, will also work. For instance, they don't list any old Roads & Rec maps, or Metro Guide North America (USA), but it can use them, just fine.
GPS programs that aren't MapSource, or MapSource compatible, generally cannot download their MAPS to any Garmin GPSr, but can upload and download waypoints, routes, and tracks.
If there's an unlisted program you're thinking about. Just list it here, and someone will let you know.