Remember when Xykon killed a bunch of self-righteous Paladins...

[Fyraltari]

Sure, but Vampires are explicitly free-willed too and yet they are inherently Evil.

Hey look, a lawful good vampire! And would you look at that, an undead doctor making sure people don't endanger themselves!

Because they, like Liches and other undeads, are quite literally made of Evil Stuff (Hell, when Good Durkon takes over Greg, he “c'n feel tha negative energy squrimin' around.” and knows it probably won’t last long).

Negative energy isn't evil, they're different things in D&D cosmology, the Negative Energy Plane has nothing to do with alignment.

I personally believe that vampires and liches are free-willed the same way a Fiend or Celestial (or a God even) is. They can make their own choices and all, but within certain limitations, because they’re still fundamentally Good or Evil or whatever, since they are made of Good/Evil/whatever stuff.

They're not free-willed. They can't help but do the morally right/wrong thing. Unlike the undead.