I don't think there's any redefinition here, and it's exactly this dichotomy that makes this a big issue. Vulnerability is indeed not "a good thing", but the issue is that the struggle to constantly keep yourself invulnerable at all times is a "worse thing", leading to many stress-related issues (amongst other problems). So the modern psychological advice, as I understand it, is to find particular people, spaces and opportunities where we can let our guard down, even at the risk of being open to attack, because the alternative is worse.

There's a stoic quote I love:

> our ideal wise man feels his troubles, but overcomes them

- Seneca, Moral letters to Lucilius/Letter 9 https://en.wikisource.org/wiki/Moral_letters_to_Lucilius/Let...

The way I see it, if you never let yourself be vulnerable, you can never fully feel your troubles, and you cannot fully overcome them.