Hi Sandeep ! Your vapour is most likely oil vapour. When the engine gets going the oil actually forms a fine mist which makes its way up the pushrod tubes out your vented covers and into your breather box. One engine builder made a rocker cover out of plastic and watched what happened as the RPMs increased and the rocker box fills up with oil!! Add a little engine heat from the valvetrain and thats what you get . Nothing to worry about. If your breather box fills up with oil,then it is more of a worry.
