A method and system for vertical handoff of a mobile station from a first-protocol RAN to a second-protocol RAN. The method and system provide a make-before-break vertical handoff, by having the first-protocol RAN (i) acquire, on behalf of the mobile station, a traffic channel assignment defining one or more traffic channel parameters for air interface communication in the second-protocol RAN, and (ii) pass to the mobile station, via the first air interface protocol, the one or more traffic channel parameters so that the mobile station can then readily switch over to communication under the second-protocol RAN. Optimally, the acquisition and passing of the one or more second-protocol traffic channel parameters may occur without the mobile station having requested the handoff, without the mobile station having asked for the traffic channel assignment, and without the mobile station having yet begun communication with the second-protocol RAN.