Network mobility (NEMO) basic support protocol enables seamless mobility in vehicular networks. In this paper, we analyze the session handoff probability in NEMO-based vehicular environments. We develop analytical models for the session handoff probability in two vehicular scenarios: 1) onboard time is deterministic (as in a subway) and 2) on-board time is variable due to traffic condition (as in a car). Numerical results are given to illustrate the effects of on-board time, cell residence time, and session duration.