CW’s “The Vampire Diaries” got the blood pumping in TV viewers, scoring the network’s largest audience in history (4.8 million viewers) on Thursday night. It topped last year’s “90210.” But not everything was rosy for the CW as the fifth-season opener of “Supernatural” did not hold on to the great lead-in. It drew 3.4 million viewers, a loss of 500,000 from last year’s premiere.