Notes

As usual disclaimer I'm not expert at all but I like to try typing so here is my thoughts:
I think Vivi is INFJ.

I agree that she has aux Fe and tert Ti (which makes sense if people confuse her between ISFJ or INFJ, Both INFJ and ISFJ like to help people but they are doing so in different way. INFJ will participate in groups and do so faithfully, but they’re happiest dreaming up original ideas that help people. They’re imaginative thinkers and visionaries who get energy when starting something new).

What makes me think she is more an INFJ is her dominant function appears to be introverted intuition, which makes her to be an imaginative thinker (She likes to help people, she likes to contribute something useful, but she does it in her own way. Even when she said she might not know well what people like in details, she'd use her imagination and sees the big picture to what will make them contented. Exhibit A: The way she picks donut cushion for Hyunjin instead of earrings.
Exhibit B: The wits she used to borrow money from Director to buy food for members). The more episodes I watch the more she gives an impression of a witty person for me. I wonder if people know this but there was a predebut interview going around, Vivi was answering in witty and savagery manner to a guy who tried to /flirt/ with her.

These are some INFJ description that I think match Vivi:

INFJs are gentle, caring, complex and highly intuitive individuals. Artistic and creative, they live in a world of hidden meanings and possibilities. She is great at drawing! We saw her works. She has good sense in fashion (look at her airport scene and her style in New Zealand story, also she talked a bit about fashion in that episode).

INFJs have uncanny insight into people and situations. They get "feelings" about things and intuitively understand them. As an extreme example, some INFJs report experiences of a psychic nature, such as getting strong feelings about there being a problem with a loved one. (I believe she is the first to notice when something is bothering her closest one for example Haseul).

INFJ can seem mysterious and private to others and I dare to bet how many of us do really know about Vivi except that she was a model predebut? Sure she was shy and not got to talk a lot but I still think she doesn't talk about her deep feeling much compared to others.

INFJs often appear quiet, caring and sensitive, and may be found listening attentively to someone else’s ideas or concerns.