Then, again - she didn't, actually, killed Vecna.
Why?
You see, Lady of Pain always was more plot device than actual character
Thus, her nature was left deliberately vague: there is just no actual story for her
Unfortunately, such vagueness leave limitless field to speculations - their rebuttal is: "Prove she isn't!.."
When I read the theory: "Lady of Pain is six squirrels in a dress with Ring of Levitation", I decided to try my hand there too
Instead inventing something completely outlandish, I decided to attack one of rather well-known - but, nevertheless (AFAIK) not directly stated in a RAW form - presumptions: "Lady of Pain is not a deity"
Oh, she don't likes her followers?
Well, guess who else don't like their followers?
Ao!
Is he not a god now too?
My real point there is:
When we take some great mystery, about which almost nothing is known - every new bit of info is chipping from that mystery.
Let's compare:
"SCP-055 is not a sphere."
"Lady of Pain is not a deity."
Thus, unless some book states in no-nonsense terms "Lady of Pain is not ...", we can speculate unrestrained:
Squirrels?
Daughter of Poseidon?
Sure, why not?!..
"Prove she isn't!.."
Pandorym is able to destroy the Multiverse;
Lady of Pain is a part of the Multiverse;
Thus - Pandorym is able to destroy Lady of Pain.
And Dark Powers of Ravenloft too.
(And Ao...)