I think plaster is a pretty good choice. It's cheap and you do need a fair volume of material to make a decently stiff form for vacuforming. You need to wear something which approximates the clothing that you'd be wearing under your armor to offset the shape properly. Casting your vaseline coated hairy bod would give you armor for you to wear nothing under.
For materials, check out:
http://www.sculpturesupply.com/
To facilitate vacum moulding, you will need air passages to remove air from all areas. A series of grooves gridded over surface of the torso form will preventing air from pocketing during the pulldown.