Right now, Wat makes for a core component to the Vader-Thrawn-Wat-(+2) GL counter team.
Use Wat’s bonus turn (zeta) to put the Weapon Tech on Vader. The ability to bypass defense will allow Vader to hit for insane damage. Other than that, speed on speed on speed with survivability is how you want to mod Wat.
I think the best way to mod Vader in this team is for potency and Crit Chance, with as much speed as possible. The stacking DoTs will ultimately do more damage than any damage-boosting mods can, but ONLY if they land, and the increased CC to help make sure you DO crit.
Honestly, just playing around a little bit and you’ll quickly learn how to defeat Padmé and DRDM. For Padmé, you want to prevent opening Protection Up on JKA so Vader’s debuffs can land and you can Cull him out of existence. DRDM is trickier as you’ll need to at least outspeed Badstilla and prevent her CBM debuff, while preventing fear on Vader so he can get to her.