  We consider Z2 spin liquids on the kagome lattice on the verge of a valence
bond solid (VBS) transition, where vortex excitations carrying Z2 magnetic flux
- so-called visons - condense. We show that these vison excitations can couple
directly to the external electromagnetic field, even though they carry neither
spin nor charge. This is possible via a magneto-elastic coupling mechanism
recently identified by Potter et al and Hao. For the case of transitions to a
36-site unit cell VBS state the corresponding finite ac-conductivity has a
specific power law frequency dependence, which is related to the crossover
exponent of the quantum critical point. The visons' contribution to the optical
conductivity at transitions to VBS states with a 12-site unit cell vanishes,
however.
