Given the wording of the feats, I'd think you wouldn't gain combat advantage until after your second attack; the first would give vulnerability, using Lasting Frost, and the second would grant combat advantage with Wintertouched
Anyway, by the time a rogue reaches paragon level and is eligible for Lasting Frost, he already has access to the encounter powers Sand in the Eyes, Topple Over, Setup Strike, and Dazing Strike, as well as the daily powers Easy Target, Blinding Barrage, Walking Wounded, Knockout, and Crimson Edge, all of which grant combat advantage for at least one turn after a hit.
There could be some advantage if you were multiclassing with a wizard or something and took an at-will power with the cold keyword, but that'd mean spending two feats and taking Wizard as a paragon path in order to gain combat advantage on a single enemy after two rounds of subpar (for a Striker) at-will attacks.
EDIT: Erisea beat me to it, and it'd only be one round of subpar attacking, since Dragon Breath's a minor action (whoops)