ROTJ: Cape collar is outside the neck. This is also the only 1 of the OT films that had the "hook" on the armor.
From ANH & ESB the cape collar was under the neck (for the most part), with the cape chain around the neck under the helmet.
I think this a great "personal preference" Vader figure that Sideshow is doing. Do what you want with the way it looks. "Out of the Box" or "Customize-rs" will have fun with this.
Of course this opinion has nothing to do with the whole Price Vs. Accuracy/what-we-should-be-getting debate.
As an action figure, strictly speaking, I think this looks awesome!
As an accurate portrayal (for me) I wouldn't be able to help myself from making changes.