That is good progress, however I have some suggestions, some which you may not like tho.
1 Are you able to handle Vixen's wings without her freaking out? Not for long, but you need to able to do that in order to get the harness fully on. Practice extending her wings one by one ( with treats each time).
2 Gradually and I mean by an 1/8 of an inch every few days, start to tighten up the body loop so that when its done, it sits loosely OVER her wings. Practice putting it on like that. We do that every single day, with Salty, and he's accepted a fully installed harness 4 years ago. Its just good practice so they stay totally OK with putting it on.
3) Heres the one you wont like. I think that while the harness you have may be the one for cockatiels, the head loop is so small that she is not liking the taking off of it, her crest feathers are pulled against the grain. Part of training for a harness is also taking it off and the parrot should do most of the work. I suggest you get one 1 or 2 sizes larger to do this initial training phase. Same color too. You want her to withdraw her head easily for the head loop, not having to push her head thru. Once she is comfy doing that, you can go back to your original harness, and maybe sell the larger one on here. Once the larger one is on though, I seriously doubt she would be able to get out of it after every thing is fully on and tightened up. Salty's harness is one size up and hr could never get out of it.
4 Develop specific commands for putting it on and off, and use those religiously every time. Thats so important in training any parrot for tricks and other stuff. Doing that lets them know exactly what is expected.
5 Once you finally get the harness completely installed, take Vixen outside immediately so she learns that the harness means going out into the world!
Good Luck, Vixen seems like a very nice parrot, who clearly wants to please you, which is a big step!
