The secret is the little notch at the beck of the neck just under the suit collar. Align the groove on the swap out piece with that notch, and push the collar down to make it snug. it isn't perfect, but it works. Unless you're going to have him fight Wolverine or Batman figures,lol.
For (one of) the Vinneys I bought, I used a spare NECA Lee Dracula body, trimmed and glued the open coat flaps down, drybrushed a little of Ole Man Vinney's hair with some lighter gray to bring out details, and plopped his butt in the NECA Pale Man chair from pan's Labyrinth.
My picture taking skills stink, but you'll get the idea.
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