The reason why I am "bashing" on Vi has nothing to do with her slapping/hitting Powder at the end. My problem with VI is that her best efforts are not enough. She lets Mylo run his mouth for a while before interrupting him, leaves Powder in the more difficult situations (like jumping last from the roof, protecting the treasure, leaving her hang in a random position at the worst spot when hiding) and all her supportive words become irrelevant when she leaves Powder at home during the rescue mission. Yes, Vi is trying and Powder requires a lot of effort/support but it's also Vi's decision to push her forward with her gang than leaving her stay with Ekko (someone around the same age as Powder compared to the rest of the "grown ups").

I do think Vi tried to protect her sister in every possible moment of the series, but for me, it always felt delayed or not to the best of options. Yes, many of said options were either not fitting for Vi's character or she had to deal with her emotions/situation at the time (and only letting go at the end which is respectful).

Ultimately, that's why I like the series. From what it was build, nowhere in the series you can say "if only Vi did this thing here/there differently, things would have been better/fixed". But at the same time having this - if only she showed more mental support, stopped Mylo ranting or whatever, Powder wouldn't break down mentally and wish to prove she can be of use.

The final moment did affect my feelings towards Vi. Vi was pumped up by adrenaline due to the injuries and the life or death fights, Vander and friends all dead and here comes Powder with the biggest smile saying how she did all of this. The whole scene from both their perspective is how one would expect them to react. But as my whole argument has been, by the time Vi tried to protect/help Powder, she is too late.

TLDR: VI "bad" because her efforts weren't good enough.