Vanessa A. is one of the very first female human characters with a prominent and antagonist role. Unlike most of the villain's of the FNaF universe who were rotten to the core, it was implied that Vanessa was once an innocent woman until she was corrupted by Glitchtrap.
She first appeared in Help Wanted as a playable character with her identity confirmed in the Dreadbear DLC, but she wasn't seen until Security Breach.
It's not made clear on where and when Vanessa got corrupted, though Help Wanted and the AR emails imply that it happened before either of those games.
After hijacking FE's system and transferring herself in the pizzaplex, Vanessa
In the "Vanny Redeemed" ending, which can be achieved after playing the previous two Princess Quest games and completing the third one,
The name "Vanny" is a mashup of "Vanessa" and "Bunny", which initially lead to speculation
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Fazbear Entertainment has been hinted in several ways that they are phasing out some of their previous characters in favour of new and unpopular characters. Bonnie is the most prominent case as he used to be exist in the pizzeria until he was presumably destroyed by Monty.
However, it's possible that Bonnie wasn't the first to be phased out. In the game, you can find a statue of Foxy in Kids' Cove, yet there is no mention of Foxy being made into a Glamrock nor he was acknowledged by the other characters. In Ruin, there were plans for a Foxy themed log ride, but it was never completed and cut outs of the character can be found through out the place, including Security Breach.
If Monty replaced Bonnie due to the character being destroyed, and Roxy replaced Foxy for an unknown reason, then it's not revealed who is planned to replaced Freddy.