Also pretty much like @Shina010615 said she's pretty much a completely new character separated from Vincent. That's why the game just calls her Van Gogh. It's similar to Molay where just the concept of him was used and the outer gods created a female body out of it. Giving us a female Molay but she's not exactly the same Molay of history.
The justification is that a jpeg of a waifu is worth more than a jpeg of a husbando and it's easy to hand wave a genderswapped version of a historical figure into existence because their core demographic just wants waifus to fawn over anyways.
I'm glad some people here were willing to go into a fun discussion about the narrative reasoning, which in my opinion are interesting reasons for the character's existence and circumstances. I appreciate conversations and people like this greatly.