I am trying to do something similar. An image analysis I complete with regions (from label_region), then I am trying to obtain the contour of each region and export them to GeoJSON with attributes. (No ENVI involved, only IDL).

This isn't directly possible with the IDL CONTOUR function, but there is a sneaky way to can do it using the procedure and the buffer device: https://www.nv5geospatialsoftware.com/Support/Maintenance-Detail/ArtMID/13350/ArticleID/15959/Sneaky-way-to-get-contour-paths-from-CONTOUR-funtion