So how does WL sanitize the vials for yeast? Dry heat??
I think they come sterile out of the manufacturing process. I doubt there is much that can live in hot plastic.
UV is an option too.
I used to do some work for one of White Labs' suppliers of the preforms -- the preforms are (somewhat suprisingly) the precursor to a two liter bottle. On the older tubes, you could take of the band that remains around the screws once the cap is off, revealing a capital "B" for Ball Corporation. Ball has since sold the plastics manufacturing division, but I can't recall who they sold the division to.
From spending a little time in a couple of the the plants, I would be surprised of Chris White hung his hat on the cleanliness of those plants. I'm sure the sterilization process for the preforms follows (at a minimum) the sanitation protocol used for filling glass bottles received from a glass plant.