Acoustic communication and vibrational communication are notable technologies as means for exchanging data among smartphones in close proximity; they can be easily implemented as software with the use of hardware mounted on smartphones. We proposed a hybrid (acoustic-vibrational) communication system, which enables to exchange data fast and securely by transmitting an encrypted message and the encryption key as sound and vibration, respectively. In this paper, we designed a software modem of the hybrid system, implemented the modem in smartphones, investigated some parameters of vibrational communication, and evaluated its security in experiments. The obtained results suggest that the software modem can realize fast and secure communication by selecting appropriate parameters for each device.