Oh, I helped a friend make her Tannis cosplay back in October.
For her jacket, we just free handed a coat pattern because her coat is kind of like a small, rectangular cape that has a semi-coat tail with sleeves and a collar. I'm not sure what kind of fabric it was, but it was not too heavy so it didn't choke her. Then we just sewed some pleather patches/straps to her coat.
For the vest inside, we used this stretchy, thick brown fabric and cut out a half tank top-like pattern (to describe it better, we cut out a whole back piece and two front pieces, like the back piece, but the neck was lower and we cut it down the center so we had two sides. But the left piece was wider because her left vest part overlaps the right side).
And then you just put the liner fabric inside and there you go. Good luck!