You put a thrawn team in. Vader inflicts 4 debuffs, whether they resist or not thats 80% turn meter gain for the opponent and youve just given the them victory (pretty much), as is, its pretty much an automatic loss on defence.
compare that to am average loss of 12.5% thats not very much and the damage probably isnt worth that drastic increase in turnmeter. Commander Luke isnt so bad with the current Vader build you have, but counter chance then takes over. which might be helpful because of the stacking to offence, but vader might just be overwhelmed.
There are 3 potential ways an edit could fix this:
a) You have it such that turnmeter cant be added based on this attack
b) You add another ability skill (not a unique), which gives vader more versatility all round and gives him options when he cant use his AoE (and obviously culling blade is situational only)
c) It makes their turns meaningless, ie a stun , ability block
But as people start to develop thrawn, Vader I feel will see less use, because he just cant do anything, except his basic which as everyone knows hits like wet noodle