While Vivian is the youngest of the Fazbear Kids, she has existed long before any of its members were created.
She first appeared as a baby in some posters alongside her parents in 1989. Her animatronic form was being constructed long before she appeared in posters for the 1989 Fazbear Christmas special.
Vivian ended up being a massive hit with audiences and spawned tons of merchandise. But despite her popularity, Fazbear Entertainment Inc. didn't consider turning her into animatronic attraction until their attempts to rebrand Freddy Fazbear's in the 1990s failed spectacularly. Up until that point, her animatronics were only being used for movies or commercials.
Much like what her father used to have before he became a member of the main band, Vivian had her own show stage which was dubbed the Witch Cove. It originally looked exactly liked Foxy's old stage but it would develop it's own personality as time went on.
In order to draw in more customers, they kept her secret, advertising her as a mysterious additional member to the Fazbear Kids crew. Luckily for Fazbear Entertainment, this ended working in their favor as they garnered national attention over it. Even new networks decided to report on it live since they really had nothing else to do.
And in addition to drawing them in, fans were overjoyed to finally see Vivian officially debut as an attraction in Freddy Fazbear's Wonderland.