Due to the way the game handles objects, Player and Player-in-vent are two separate entities, which we can abuse for skipping the "Kill backup" objective.

When the enemies arrive, they are given Player's current coordinates to move forward to. When we are in a vent, however, the Player object is nonexistent and, as such, the backup is given coordinates to a nonexistent entity. As a failsafe, instead of crashing, the game skips the objective and lets us avoid wasting time on killing more undead.