To alleviate the shortage of wireless spectrum for vehicular communications, integrated sensing and communication (ISAC) technology is expected to be applied to vehicular networks. In this paper, we design a dual-beam ISAC scheme that allows 360° radar detection and directional communication simultaneously. We then develop a stochastic geometry-based performance analytical framework to evaluate both sensing and communication performance of the dual-beam ISAC vehicular network. The detection probability and communication coverage probability are modeled by capturing the impact of both the incident and reflected interference. Furthermore, to improve the sensing performance of 360° radar detection while guaranteeing communication performance, we study two power allocation optimization problems. Simulation results not only validate the accuracy of the proposed analytical models, but also show the advantages of the proposed power allocation schemes in improving average detection probability.