A themataic map usually implies that you have classified a multispectral image. So you would need to determine what criteria you would use to find areas of 'moisture' and run a classifier based on training data you selected that represent 'moist areas'. Classifiers are found under ENVI's Classification menu. Most ENVI menus allow you to subset data by an ROI or Vector which would allow you to restrict your classification to a chosen boundary. You could do this prior to classification as well using Basic Tools ->Subset data or convert your vectors to ROIs and use Basic Tools ->Subset data via ROI.

Hi Marim! Thanks for the reply. An example of expected result is shown below. I have satellite image who follows the same color gradient and I want the image to be displayed in shapefiles according to certain boundary shapefile. So, I am not sure about the classification. Please do enlighten me on this matter.

If your image looks like the color gradient above, then it is already a thematic layer and not a satellite image representing reflectance. Calculate statistics on the data (basic Tools ->Statistics). If it shows defined values of 1-say 10 (e.g. it has 10 classes), then it has already been classified and you can the Raster to Vector tool under the Vector menu.