Paul
regarding materials for the vestibule. I have a Bibler Eldorado which is where your tent design originated. The vestibule takes a real thrashing in a bad blow and icing in the top is a drag if you have to use the vestibule to get in and out. Use the most rigid material and the one that may ice up less. Holding its shape will be critical if you have to cook in it in a storm.

Also in a tiny vestibule, digging down can give you a place to sit in the front of the tent while pulling on your boots in a "sump" in front of you and give you more clearance for a stove.
Jim