Vortices in conventional superconductors are typically dissipative, yet recent work [Nambisan et al. arXiv: 2510.19769] showed that vortices trapped in granular aluminum (grAl) can form coherent quantum states with microsecond coherence times. Although these states can be coherently controlled and read out in a circuit quantum electrodynamics architecture, their microscopic origin remains unclear. Here, we use in-plane magnetic fields to probe their susceptibility and place new constraints on possible microscopic models.