My personal definition is this.
Any Truck, Car, Jeep, Armored Car, or Halftrack, I consider a "softskin" as they were not designed to go head to head with Tanks.
Any Tank, or Tank Hunter, or Tank Destroyer and any other med to heavyly armored vehicle designed to fight tanks head to head I consider "hardskins"
of course this is my own personal way of defineing them.