I do teach my flyers not to catch themselves when learning twist downs/ double downs, but to put their arms straight down over chests and stomach so it does somewhat protect them if they happen to land on their stomachs.. but even so.
Huh!? Sorry, but you're protecting their ribs at the expense of their head and neck? They need their arms to catch themselves. I hate seeing all these supposed double downs where they just "spin" and don't even land in a proper cradle. If I was judging, that wouldn't count as a double down, just like landing a full with your body squared up but your feet landing at a 90 degree angle isn't a full.
I don't know about the vest either. I prefer progressions for my team before ANY kind of spotting, matting or devices like this. But I can see it being helpful for practicing them over and over, like a spring floor is helpful for practicing tumbling over and over. I'd just be careful not to substitute something like this for good progressions.
And have your flyer catch herself in a cradle.