Medical help rushed in as Estrada was prone on the canvas for several minutes before being allowed to rise slowly.
Time of the stoppage was 33 seconds into the round.
Vargas is now trained by Roberto Alcazar, replacing Roger Mayweather, whom he had worked with since age 15.
In the televised-co feature bout, Venezuelan super welterweight Alfonso Blanco, 6-0 (2), defeated Cleveland Ishe, 3-1 (1), in a six-round outing.
The posing, unorthodox Ishe gave Blanco fits in the first round, as the latter was unable to figure out Ishe’s style, often getting beaten to the punch.
Blanco saw a better second round and was able to let his hands go in combinations. Ishe landed hooking shots before getting caught to the chin and driven into the ropes.
Blanco finished the round strong by digging to the body. Blanco’s relentless pressure had Ishe moving backward while trying to get a breather by the end of the third round.
Ishe became a sitting duck in the fourth and fifth, getting clobbered from post to post, resorting to throwing single shots and mainly missing the mark.
Ishe became the “Drunken Master” in the final round, going back to the wild style of the first round, with Blanco trying his hardest to finish the bout.
Scores were 60-54 (twice) and 59-55 all for Blanco.
Cruiserweight DeMark Scott, 3-4-1 (2), lost to William Bailey, 11-11 (4), who lived up to his “Billy Goat” moniker- head first and punches following- making life hell for Scott.
Bailey did as he pleased, giving Scott problems for the entirety of the bout. Scores were 39-37 (twice) with the third judge scoring it a draw.