The key to making vacuform armor look good onstage and not like "OMG plastik kostoom arm0r LOL" is covering it before painting it.
For less detailed pieces (plain breastplates, smooth greves, etc) you can use felt--industrial felt if you want it thick, or craft felt or oak-tanned leather if you need it thinner. With detailed pieces, such as our lion-head-relief shoulder-guards, we're using pigsuede--it's stretchier and easy to manipulate into curves and crevices.
( Here's some photos of how that works. )
For less detailed pieces (plain breastplates, smooth greves, etc) you can use felt--industrial felt if you want it thick, or craft felt or oak-tanned leather if you need it thinner. With detailed pieces, such as our lion-head-relief shoulder-guards, we're using pigsuede--it's stretchier and easy to manipulate into curves and crevices.
( Here's some photos of how that works. )