I would guess it would be superficially similar though with less of a power imbalance between the two due to age Tootie would be less of an easy target for Vicky. There's some possibility Tootie may even retaliate occasionally, but I think she'd lose far more often than she'd win, Vicky's shown to be able to wear down even those nominally her equal. One notable difference is that Vicky would probably mix in more social/psychological bullying along with the physical ones.
Now with that said I think it could go two ways,
A) Similar to canon- Vicky is very much the bane of Tootie's existence and her main target after Timmy, though with Vicky's relationship with Timmantha in this AU it could either end with Tootie being the primary focus of Vicky's sadism or Timmantha distracts Vicky enough Tootie's left alone which could add a complication with Tootie wanting Timmantha
B)Tootie's still a target of opportunity, but not a main one anymore, possibly even not much more than standard older sibling/younger sibling style bullying. Vicky's gone for other, easier targets to bully, she's still a bitch, but not horribly so to Tootie. Tootie's feelings towards her are likely more along the lines of dislike than outright fear, with the possibility that if their relationship is not too heated, Tootie might feel some degree of admiration towards Vicky, while Vicky for her part would probably find Tootie less annoying, though that may simply be due to her not being 9-10 than less of an age gap
Personally for the purpose of this AU I'd go with B, lets things like >>91126482
's ideas make more sense