IMHO Wintertouched is just like Distant Advantage: it's a feat you should discuss with your party.
If the Rogue and the Fighter both have Frost weapons, all three of you should take Wintertouched + Lasting Frost, since the probability of Wintertouched being useful skyrockets when there are multiple sources for vulnerability.
Cheers, -- N
Huh? You need to take a Frost Weapon/cold powers as well for the feats to do any good!
Wintertouched
Benefit: When attacking a creature that is vulnerable to cold, you gain combat advantage when you use a power that has the cold keyword.
Lasting Frost
Benefit: Any target you hit with a power that has the cold keyword gains vulnerable cold 5 until the end
of your next turn.