Good post Chris and Nick.
It has always been confusing for a lot of people differentiating the difference between vapor barrier and vapor retarder.
One way, or another way to help is as listed here;
The current code has the following definitions:
Class I Vapor Retarder: 0.1 perm or less
Class II Vapor Retarder: 1.0 perm or less and greater than 0.1 perm
Class III Vapor Retarder: 10 perm or less and greater than 1.0 perm
The current proposals are to define them this way:
Vapor impermeable: 0.1 perm or less
Vapor semi-impermeable: 1.0 perm or less and greater than 0.1 perm
Vapor semi-permeable: 10 perms or less and greater than 1.0 perm
Vapor permeable: greater than 10 perms
More on the subject here;