Are there any maps available that would be useful for walking off-road in southern spain that I can use with my Garmin etrex vista HCX?
As an extension to Openstreetmap, I recommend Opentopomap. It has the same trail data as Openstreetmap, but adds contour lines, which can be quite handy when hiking.
However, I recommend to never hike on a GPS alone. Bring a paper map from the Spanish IGN. You'll get more overview, and a GPS can break or run out of battery.