Hi jermy4,
According to the Costco description here and here the Vellux blankets are polyester.
Hopefully some of the other forum members will see your post and share their feedback about the blankets they are using since there isn’t a blanket thread in the forum (although there is a reference topic about sheets and different types of fibers and fabrics here which would have some relevance to blankets as well).
I just asked my DH what kind of blankets we have (there isn’t a label on the blanket) and she told me they are also from Costco. They are also synthetic polyester.
I asked her “how do you expect us to have any credibility if we use synthetic polyester blankets?” and she laughed … and “promised” that she would change our blankets for cotton as soon as she “got around to it” 
Of course I was “appropriately disgusted” :).
Phoenix