How worth is a Zeta for the ship?

I believe it will add about 5k GP to the ship.

I would recommend to zeta Padme instead, if that one zeta gives you a whole new team to use in TW/GAC.

Waqui... But...does it increase the stats of the ship in a considerable way? A worthy way?

let me explain you my situation:
Currently, I am around 100th position in Squad Arena. Usually being able to finish in 95 ish position...
I have Padme Ahsoka and GMY at 7*...
But Anakin, Gen Kenobi will take yet around 6 weeks to be 7*...
3PO is far yet for me (although Ewoks are being upgraded now too)

On the other hand, in Fleet Arena, I ve been able to finish in 3rd place already.
usually sitting around the 5th easily
I am using
Executrix
Geo Trio ( max abilities and Toons at R4)
Hound's Tooth (yet 6*)
Tie X1

So... would giving the last zeta to Vader drastically increase my chances in Fleet Arena?

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.

Ship stat increase for a character zeta is minor. Roughly equivalent to a piece of g12+ gear.

So not worth adding a character zeta for the ship stats unless you absolutely have to max that ship's speed and you know from a calculator like crouchingrancor that that particular zeta will add a point - wont always be the case.

I would add gear, relics and 6e mods for ship stats long before I would consider adding a zeta just for that purpose.