How worth is a Zeta for the ship?

Adding a zeta to a crew member like Vader will also increase his crew power and hence also his ship's GP and stats (depending on how much the crew power is increased). If the only purpose is to increase your ship's stats I would only recommend to spend zetas on crew members on key ships f.ex.Capital ships, where speed is essential to reach that ultimate before your opponent, or maybe also HT for increased tankyness. I wouldn't recommend it for tie advanced - especially since it has limited use long term. I would only recommend to zeta Vader's leader ability if you're planning to use him as leader in other game modes. Since you're using HT and it has far better long term viability you would benefit more from zetaing Bossk instead.