Recently, I found these socks, which are vegan and at least as warm as, if not warmer than, wool:
These crew thermal socks not only insulate but use the super moisture-wicking properties of bamboo viscose to keep feet drier – the thick inner loops of cushioning trap and hold heat, keeping your feet warm and dry for hours.
www.ecosox.com
They are pretty amazing. At -25 or -30 C, you will probably need to layer them and wear them with warm vegan boots like these for example:
Get ready for an outdoor adventure with the Veggie Trekker! These boots are amazingly sturdy and great for hiking and braving the elements. Microfiber faux leather and rubber outer and sole Rubber on bottom provides water-proofing Wind-tex breathable and water-resistant membrane lining and...
mooshoes.myshopify.com
PAWJ California | Shop our line of 100% Vegan and Cruelty-Free Boots, Shoes and Slippers.
pawj-california.myshopify.com
(or, if these are too expensive, something from a thrift store).
I have been getting frostbite on my toes in the winter even though it doesn't usually get below -5 C or so where I currently live. I've been thinking that it may be caused by poor circulation. I've been making sure to drink close to 2 L of water every day, and that seems to have helped, at least so far.