I'm 99% sure Barbara didn't pass the castings and that's why she didn't walk, around a week or two before the castings she posted on twitter that she was hoping to make it to the VSFS this year. No sane agent would book you a photoshoot the same day of the VSFS unless it was something super major and looking at the crew Barbara seems to be working with in China, it doesn't scream major to me.
She wasn't in NYC for the castings but she was there for the callbacks, it was the same situation with Sara Sampaio, she didn't attend the castings but she did go to the callbacks. And Barbara didn't even have to go to castings last year so If VS really wanted her there they would have let her walk anyway just like Cara did.
I have my theory as to why Barbara maybe didn't pass the castings and I've discussed it with other people and it seems like it's the overall feeling.
I do love her VS work and I hope she starts working with them again and hopefully she'll walk next year and have a big relationship with them.