I would leave Vit’s basic function ‘as is’, the game still has a role for Vit in content where pet stasis is applied. (Eg. I find sorcerer’s max 75 makes me more resilient in these places than when I’m playing on lesser Vit classes, or when I’m on Vit unmaxed.) If players choose to ignore this and run unmaxed, that’s their loss when they get petstasised. Sidenote: I probably would concede giving Vit an increase in power, if pets aren’t going to be nerfed, to help reduce the chasm between weak pets and strong.
But yes a 9th stat that makes players more resistant to debuffs, sure. Though I think it wouldn’t work to be solely changing the durations of effects, since much of our experience is on timing, and having that become fluid would screw with our playing. (Eg I know rushing WC if I get hit by a confuse instinctively how long to use confusecontrols for. That would be lost if a stat started to affect the time.) But that’s just confuse - others might be fine, eg slow could be reduced to ‘not so slow’, cursed/bleeding/dazed could be lessened, so there’s probably scope for it to work.
Would have to be careful not to make it just be a overall playerbuff to allow a 9/9 player to be super resiliant though, and have a legion of superplayers walking through all the content, tanking effects without a care.