Why boundary between liquid and gas phase disappear on heating a liquid up to critical temperature?(in a closed vessel). What is the state of substance at this stage

On heating vigorously and continously the temp. increases and thus vapor pressure increases and at the first phase of line boundary the liquid has more density than the gas. And on more heating all liquid gets vaporized with vapor gases left having high density and the molecules  are being get apart. And lastly at the final stage when both liquid and gaseous are in equilibrium the line gets dissapeared. This is called obvious as critical temperature.