In Virtual Reality (VR), a growing number of applications involve verbal communications with avatars, such as for teleconference, entertainment, virtual training, social networks, etc. In this context, our paper aims to investigate how tactile feedback consisting in vibrations synchronized with speech could influence aspects related to VR social interactions such as persuasion, co-presence and leadership. We conducted two experiments where participants embody a first-person avatar attending a virtual meeting in immersive VR. In the first experiment, participants were listening to two speaking virtual agents and the speech of one agent was augmented with vibrotactile feedback. Interestingly, the results show that such vibrotactile feedback could significantly improve the perceived co-presence but also the persuasiveness and leadership of the haptically-augmented agent. In the second experiment, the participants were asked to speak to two agents, and their own speech was augmented or not with vibrotactile feedback. The results show that vibrotactile feedback had again a positive effect on co-presence, and that participants perceive their speech as more persuasive in presence of haptic feedback. Taken together, our results demonstrate the strong potential of haptic feedback for supporting social interactions in VR, and pave the way to novel usages of vibrations in a wide range of applications in which verbal communication plays a prominent role.