Most of it doesn’t apply or is overpowered now, but vulnerability to turning is still viable….my only peeve being how in hell do you word it?
There are no longer any monsters in the MM, or elsewhere, that have vulnerability to turning, i.e. take more damage or something.
Normally this should be filed under the heading Vulnerability (just like Immune or Resist)...but vulnerable to what?
“Vulnerable 10 channel divinity: turn undead” ???

Probably vulnerable to radiance damage would be the way to do it.

....simple, yet works. Vulnerable 5 should be enough, you think?

Vulnerable 5 Radiance should be enough for any situation, also remember any Vulnerability that is added later is cumulative, and there are a few of those floating around.

What do you mean, a vulnerability added later?

4e has a few different powers that add vulnerability to one thing or another, radiance in particular from paladins is not uncommon, and rather low level, there are others, and they all stack. So if someone’s plan is to make targets vulnerable to radiant damage that plan will work well on the grey touched and not be redundant, or even really overkill.