In vehicular communications, one of the key requirements of the vehicle-to-vehicle (V2V) links is to transmit a given amount of data successfully during a given time period. To this end, this letter investigates a spectrum and power allocation problem in vehicular networks to maximize the sum ergodic capacity of the vehicle-to-network (V2N) links subject to the V2V links' data amount violation probability. By using the large deviation theory, we express the data amount outage probability in a tractable form and derive the optimal power allocation for each possible spectrum reusing pair consisting of one V2N link and one V2V link. Then, the optimal spectrum reusing pattern is obtained by addressing a bipartite matching issue. Finally, the effectiveness of our proposal is validated by simulation results.