Pinching antennas, as a novel class of low-cost and flexibly controllable antenna systems, offer a promising solution for next-generation wireless applications. In this paper, we propose a pinching antenna-assisted integrated sensing and communication (ISAC) framework, where index modulation (IM) is employed for efficient data transmission. Simultaneously, the user leverages the line-of-sight (LoS) component to estimate its position, thereby enabling the ISAC functionality. Furthermore, to address the inherent computational challenges, the proposed scheme invokes variational inference for approximating the joint posterior distribution of user location and transmitted symbols. Meanwhile, the prior knowledge of the user’s spatial constraints is incorporated through a turbo-type iterative update mechanism, which significantly enhances the estimation accuracy. Finally, simulation results confirm the effectiveness of the proposed algorithm in terms of both localization precision and detection reliability.