The image file and the shapefile do not have to be in the same projection with the same parameters to overlay them.  They both do need to be imported with their correct map information.  For example, import your GeoTIFF and make sure ENVI recognizes its projection in the Map Info.  Then import your shapefile.  You will probably be prompted to enter the projection of the shapefile.  Make sure you enter the projection it is currently in, not a projection you want it in.  Make sure the shapefile imports correctly by displaying it in its own vector window and look at the map information of the various nodes in the shapefile.  If they look correct, they should overlay on the geotiff even if they are in a different projection or datum.  ENVI will project the shapefile from its current projection to match that of the geotiff.

Dear mminari, I got sucess  in overling my shape file following your precise instructions. I have to define a new  projection as the original projection of the vector was not in the list. Initially I was defining the projection based on the Image parameters, where i was wrong! Thanks  man