Well - maye you are new. Sound like 3 months is a long time. I probably get 2 or so a month.
If you are getting them that rarely I would max out an arena team with zetas. Maul is better leader zetaed. But if you have so few Vader may be fine now. Maybe a low star zeta on Sion would help you more. Sion works as 3* depending on where you are in arena and what your shard looks like. Maybe Tarkin even. He is good early game arena.
The problem is that if you are hitting a lot of Bastilla squads then early game sith will struggle. Now you are farming OR anyway then probably should save your zeta for Bastilla lead and switch to Jedi when you can. I think Carth is a good OR lead too but he relies on debuffs and zeta Bastilla will resist those a lot.
Need to build a good arena team, move up, get crystals and upgrade your fleet and move up in fleet. Fleet is super important to get zetas faster. I just can't really give great advice on early game arena. Maybe Google a recent guide? Whatever it is OR farming sounds smart, if you are already down the Revan path.