ABC's World News and NBC's Nightly News wound up in a virtual dead heat in the ratings last week. Nielsen Research said that the ABC newscast drew 8.08 million viewers, while NBC's drew 7.93 million. The less-than-2-percent difference is considered statistically insignificant. ABC's Elizabeth Vargas substituted for Charles Gibson throughout the week. On CBS, Evening News boosted its audience a bit, but its 6.13 million viewers was well below the number for the two other nighttime newscasts.