A couple of things that jump out to me are:
1. The bucket has too much of an "underbite" and missing some details:
501st Legion: Vader's Fist
In addition, the tube stripes you have are too long, and you're missing the flat vertical middle chin detail and the triangular cutout under the visor. I'd also look at the way you've painted the ears. I'm not 100% sure the black goes on the vertical tubes, but I know it needs to border the ear.
2. The boots are missing a detail piece up front, the side soles should be primer grey and not white, and the strap disc is too pronounced:
501st Legion: Vader's Fist
