http://www.brinkthegame.com/
I know you don't have to use the vest to use the mask, Devil

In-game Eel facemask, made me so happy :happy:
Anyways.. In a screenshot, we can see the Eel Vest without the hoodie on, hinting at 2 variations of it, like The Tank and The Tank Vest - But if you now have the Hooded Eel Vest, you shouldn't be able to use head-gear, since.. Well, having a cap ontop of a hoodie is damn silly, and a cowboy hat? I don't think you can
So that makes me think that as soon as you scroll down to the Eel Vest with the Eel Mask equipped, he gets a hoodie, and the possibility of using a hat is removed, but then there would logically only be 1 variation, meaning only The Eel Mask gets a hoodie.. Which does make sense in a way but still not
EDIT: http://www.strategyinformer.com/pc/brink/screenshot-viewer.html?id=259026 - There is the screenshot of the Eel Vest without the hoodie