So, we still have to wait a little?
My main argument was that you couldn't say Van Vleuten did it last year.
Key operator being 'if'. If we consider a GT to be defined as a race of three weeks, then no woman has won a GT stage yet. If we consider a GT to be any of Giro, Tour or Vuelta, then it doesn't matter whether the Vuelta is five or seven days long, a stage win is a GT stage win in both cases. So the only reason to consider Vos to have won a GT stage today but not Van Vleuten last year, would be to think of a stage race called the Ceratizit Challenge by La Vuelta as not being a Vuelta. And I wouldn't say that that was the prevailing opinion last year.
And as for what we consider to be a GT, and therefore who (if anyone) we consider to be the first woman to complete the hattrick of GT stages, I don't think there's a clear answer to that, and I also don't think there's going to be consensus on that. I suspect that, if we reach a point where the women's GTs are three weeks in length, we will think of records in this era like we think of records in tennis prior to the open era, in that they count in some ways but not in others. But at the same time, I don't see how the value of a stage win is lower when the GT is shorter - if anything, it makes it harder to get one. So I would argue that the first GT stage triples in women's cycling were completed last year, and not by Vos today.