Vadar wasn't the leader of the army, was he? Grand Muff Tarkin was making all the decisions. Vadar was just kind of a badass free agent enforcer type. The Emperor's consigliere. The Emperor had an agenda of wiping out the Force and any Jedi's lurking around, so it was going to be him that told Vadar to focus on recruiting or destroying Luke. Vadar did bring Luke in chains before the Emperor in the final movie, so he did succeed.

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