I get this question A LOT, as do most of the GMLs in the legion, and to put it in short....
"Don't waste your money."
To answer your question in long form:
Vader is the centerpiece of the 501st. His appearance has to be superior to all builds, and be
able to stand up to some serious scrutiny. The Rubies Vader, especially right out of the box,
doesn't even come close to the standards that are required by the 501st.
Just some of the modifications that are required are
Shins' cloth back needs to be cut off and replaced with buckles.
Shins, Belt Boxes, Shoulders, helmet all need to be repainted.
Belt needs to be replaced with Leather Belt.
Buckle needs to be replaced and be aluminum.
Helmet shape needs to be modified, making the nose more angular and correct.
Chest box rods, and brass pieces, need to be replaced with metal parts.
Electrical orientation of chest box needs to be reversed.
Cape will need to be hemmed to correct length.
These are just some, all that I can think of off the top of my head (I have a complete list somewhere,) of the required mods.