Evaluate my Vader please, mod suggestions?

Speed is always key on Vader and that's pretty fast. Until you could exceed the speed it's not worth changing much.

Who are you fighting against? If there's bastilas in your shard you'll need like 150+ potency at a minimum (around 200 is better if you want a good chance of any dots sticking to speed you up at the beginning of the match). 70% is pretty low if the enemy has any sort of tenacity buffs.

Is Vader worth modding for Offense or Crit Damage or does he always hit like a limp noodle?

I initially modded for Potency to land the DoTs and slows and Health/Protection because he was under-geared at the time.

He's TW/GA B team material so not my highest modding priority but I could work on switching things up if the potential DPS gains are worth it.

If you're using him in raids and outside arena then yes, extra damage is great. In arena, as we unfortunately know, 97.2% of the time, speed is king. A speed x4 set with the remaining two being whatever mods have your highest secondaries regardless of speed. Ideally they'd be a pair, but Vader typically needs to go first under zPalp.

I have a second set of mods for my Vader for when I max out Wampa so I can run a high damage Wampanader team. Just swap between the two depending on where I'm using him. (Not my arena team)

TB: Doesn't really matter whether you go for speed or damage, both will be sufficient when you use him under a zeta Palp.
Raids: Damage
Arena/TW/GA: Speed (+ potency). You want him to go first with his AoE + Saber throw combo in order to start the TM train. An example of the importance of speed? Palp vs BH. Bossk goes first? BH win. Vader goes first? Palp wins.