Thats pretty damn good looking. You should silicone that instead, then sell copys of it. But if you want to vaccuform it, make plaster molds of it, reproduce each half in plaster from the molds(using release agents of course), and vaccuform them, then epoxy the 2 halfs together
In my opinion, you'd really have to chop up the helmet to get good pulls - there are way too many sharp corners and undercuts on the bucket to reliably vacuform it. Not saying it can't be done, but if you look at the helmet from each angle, you've got undercuts all over the place.
That being said - here's an idea - multiple pieces.
back - from the back to the little ear indent. Including it will cause an undercut, but it could be used to line the pieces up when assembling it later
jaw - left jaw, right jaw, assembled in the middle - filler and sand smooth
top - should be fairly easy, as long as you do a visor also
5 pieces? I think if someone wants to do it, they could. Personally, I think going the fiberglass route will result in better results.