I've been thinking more about this. Fundamentally i don't think there is really anything wrong with the costume. I think if you took the R1 costume with you in a time machine and went back and dressed 1977 David Prowse in it it would be Darth Vader.
I think one of the biggest issues is how the character was filmed, mostly in the first Vader scene in R1. Lighting makes a huge difference. I've learned as someone who costumes as Vader and takes photos that the overhead light is very important.
The eyes should be in the shadow of the dome. The chin should be in the shadow of the mask. The area between the mask and the dome should be in shadow. I know in movies, even the original trilogy, they used lights coming from the camera side. But the overhead light needs to be the brightest to create shadows in the helmet.
I think one of the reasons the neck looked so obvious and gunmetal is because it was so lit-up instead of being in a shadow.