I'm not just basing my remarks on the Kermit photo and really, I'm just playing devil's advocate here and urging others to keep an open mind about this topic. My points were all made in order to keep everyone in check as Anson's post seemed to me to be a declaration that the production used shins were fiberglass and the vac formed ones were post production units. I cherish and believe Brian's knowledge as well, but we have some definate unknowns present that prevent an answer either way.
With regards to the Kermit pics and whether those shins are indeed plastic, yes, I agree that fiberglass can be laid thin. I would also contend that fiberglass that thin would be extremely brittle and an edge such as that, at the flex point of the ankle would most probably be very jagged, cracked, and/or broken after use. The pictures show a pretty smooth, thin edge.....again, based on that pic, albiet small, the shin appears to be plastic.
Mac, I apologize if I misunderstood your post, but I believe you were speaking of an undercut when you described the 'z'. Undercuts are present in many vac formed pieces. The most extreme would be the stormtrooper face plate. Removing a vac formed piece from a buck with an undercut is related to the shape of the piece formed, the thickness/flexibility of the material after vac forming, and the ability to cut the piece from the buck and the unmolded material. The part can also change depending on who is cutting the form from the buck and how it is done each time. You do not have to have a vertical line mold to the base in order to vac form a part; such a small lip on the shins would be very easy to do with an undercut. Again, I'll apologize in advance if I misread your post.
Even if the Kermit photo was not available, there is other evidence that suggest the shins could have been vac formed for production. We have all seen the PSOL shins and I would also point out that in the same offering from them was a vac formed threepio shoe in plastic. As I posted previously, both threepio's shoes and shorts were vac formed in plastic for the ANH production (no idea about color), as well as the stunt torso. Even though we are dealing with two different characters, it does reveal that for threepio, the production used both fiberglass and vac formed parts for the hero suit. More importantly though, with regards to the threepio stunt torso, it reveals that they made a vac formed part that was originally cast in fiberglass. Why would we assume that they would not do the same thing for a smaller, easier part to vac form such as the shins??
Anyway, food for thought. I hope someday some reference becomes available so that we know for sure, but until then I'll enjoy the journey. And Anson, when I wrote 'ruffled my feathers', I did not mean that your post offended me in any way, I just meant that it woke me up and made me contribute. Not a bad thing.