theres only a couple of things i know of that can cause the vaporizer to freeze up
- poor or no coolant circulation through the vaporizer
- an internal problem with the vaporizer, ruptured diaphragm

I would do as suggested previously, check the coolant circulation, including making sure the waterpump is circulating through the radiator. Also make sure the thermostat is working.

if circulation is poor then flush the system or replace any clogged hoses
if circulation good then i would suggest replacing the vaporizer (you can try rebuilding it but by the time you do that you could just replace it for about the same cost).