I have no experience with this type of work, so this is all just my opinion:
I believe that most sintra is thick, at least from what I've seen. Most heat guns will not get THAT hot. I don't think it will be possible to heat the sintra enough with a heat gun that it would become pliable enough to be sunk over a mold. I just don't think it could happen.
ABS/Styrene is much much thinner, I think maybe even less than half as thick in some cases. And vacuform tables, I believe, get VERY hot. It's the negative pressure from the vactable that does the forming and pulling. Without that pull I don't know how well it could work.
Depending on the thickness you may be able to heat it up and sort of hand-pull the sintra over the form, at least getting the desired bend and shape. I don't think I would expect it to get much detail though.
If it's for a movie and you have a good painter you might just try posterboard. It's very cheap, easily replaced, and easy to form. Just apply velcro where you want it to go and it's so pliable that it will be held easily to the vest etc. It's thick enough to hold a crease as well. If it's painted properly I imagine it would do the trick.
Crazy maybe, but certainly worth a thought,
Phil