An'ya pushed Vader only through exploiting the environment (teleporting, Plant Surge, etc.) When the two of them engaged in direct combat, An'ya tired within a few panels of dueling, was disarmed quickly, and is overall implied to be well below Vader's class.
On "neutral ground", the Dark Woman won't have access to the luxuries that she did against Vader. Given that Ben was beating Sith Sabers on Dromund Kaas, a dark side nexus that clogged even Luke's reflexes, I'm inclined to think that An'ya slaughtering is a bit preposterous. She did lose to AotC A'Sharad Hett in a duel, did she not?