Hello HHH,
Today's Hotfix updates Darth Vader's Datacron to address the issue with him and and the newer Bad Batch squad. Darth Vader will be able to be defeated if all allies are defeated or Captured.
Previous Description:
At the start of battle, if there are no Galactic Legend allies, Darth Vader can't be defeated until all the other allies are defeated. The first time each ally is defeated, Darth Vader dispels all debuffs on himself, resets all of his cooldowns, and takes a bonus turn.
During his turn, Darth Vader has +50% Health Steal for each defeated ally.
Updated Description:
At the start of battle, if there are no Galactic Legend allies, Darth Vader can't be defeated until all the other allies are defeated or Captured. The first time each ally is defeated, Darth Vader dispels all debuffs on himself, resets all of his cooldowns, and takes a bonus turn.
During his turn, Darth Vader has +50% Health Steal for each defeated ally.