Last night, I opened up my VOTC Vader and was checking out the articulation when I got to the knees. I went to move the left one because I could see it was articulated and the lower leg broke halfway off.
Upon closer examination, I figured out that Hasbro had designed this figure to have a univeral-jointed knee, but unlike most uni-joints, this one was a rotation joint at the top, a hinge joint in the middle, AND a second rotation joint at the bottom. This lower rotation joint plugs into the lower leg at the boot and in the manufacturing process, the factory workers installed the joint and turned the lower leg forwards but because the uni-joint had rotation at the top AND bottom it can rotated freely from the upper AND lower leg so it ended up facing sideways instead of foward, so when I went to bend the knee, I didn't see that the uni-joint was facing sideways and the lower leg's connection to the joint broke with almost no effort.
Examining the other knee found the same thing, the joint was sideways and now that I knew what to look for I could see (just barely) how things were aligned.
Anyway, bottom line: if you plan on opening your VOTC Vader figure, double-check the orientation of the knee joint before trying to move it.