Set Render Texture size without cropping?

I'm trying to render a screenshot that's smaller than the actual resolution of the phone. I'm setting vid_wid and vid_hei to maintain the actual aspect ratio of the phone, in this case 640x360. The problem is, I only get the bottom corner of the screen. I want the render_camera to render at a lower resolution than the other (game) camera, but without cropping. Here is how I set the target texture, (the commented lines are various ways I've tried to force the camera to render the whole screen, to no avail with all, none, or various combinations).

         if ( render_camera.targetTexture != null ) {
             render_camera.targetTexture.Release( );
         }
 
         render_camera.targetTexture = new RenderTexture( (int)vid_wid, (int)vid_hei, 24 );
         //render_camera.aspect = vid_wid/vid_hei;
         //render_camera.rect = new Rect(0f,0f,1f,1f);
         //render_camera.pixelRect = new Rect(0,0,Screen.width,Screen.height);

Help would be greatly appreciated! Thank you.