I'm not convinced there are clear domes over the vents, half the time I'm not even convinced there's a metal dome/bolt holding the thing on.
I don't see any domes here, just something backing on the inside of the mask:
These Leia shot looks pretty translucent to me:
This Han shot could show a reflection, but also looks pretty translucent:
But then ones like this look pretty metallic:
The nose is pretty shiny there too, and we know that's clear. I haven't seen anything that convinces me that there are transparent domes over the top, but there are definitely some shots where that could be the case. It's enough to make you go a bit crazy!
Edit: Now I'm seeing a dome on the image-left vent in the first screenshot, but not on the right one. The extra layer of transparency would lead to different reflections, it could look like metal in some shots. Ugh.