Vordak's character is not that tough and he is exploitable. I would rather you guys trusted my judgement on characters and their acceptance instead of questioning every time YOU feel they are unfair.
Well, I literally only have what he told us to work with, run at 80 m/s, hit with enough force to kill many humans at once instantly, durable enough to sustain sitting in the sun for five seconds, completely immune to a wide range of magics, and obviously protected from many other forms of magic by the nature of his durability.
Having a weak-spot inside that sort of structure is really a moot point. That's like saying the weak-point of an M1 Tank is the dude inside, well great, but I'm going to get fucked up before I can reach him.