Maybe because he was a little busy having an apocalyptic duel with someone at the time. Since the HoT won the fight, it's reasonable to suppose that she was able to protect T3 if Vitiate actually bothered attacking him.
Ahsoka fought a Vader that couldn't even take a holocron from Ezra. Couldn't kill him in their first meeting either. And got pressed by some random trandoshan.
Originally posted by NephthysWho was significantly weakened, evidently to the point at which he's inferior to Arcann, or the HoT just isn't as strong as he was then. Nice try tho.
The Hot defeated Vitiate. As in, actually won. And is stated as more powerful than Jedi whose tk shits on Ahsoka. You don't want to get into a dick-measuring contest here.
And lol, I bet.
One good fight against an opponent with conflicted feelings towards her, that she lost, doesn't trump the HoTs numerous superior feats and victories. Ahsoka struggled to defeat the Inquisitors. For all you go on about Ahsoka lasting so long against Vader, the Brother lasted about as long against her. 5th Bro for Maul tier? The Hero would own those fools.
It's a poor showing for Vader.