I think it's very unlikely wing tip vortices would pose a direct risk to ground objects. They are a risk to other aircraft in close proximity which are effectively being supported by the air as they fly through it, the turbulent airflow can throw an aircraft around quite a bit and possibly risks affecting the amount of lift produced by that aircrafts wings. But to an object that is being supported by the ground rather than lift created by flying through the air the effects will be much milder. I doubt it would pose any more risk than a strong wind.
There have been stories of overflying aircraft dislodging roof tiles. I don't think anything has ever been proven (tiles do sometimes just fall off even on houses not directly under flight paths) and I suspect that even if the overflying aircraft are a factor they will certainly not be the only one, i.e. the tiles may well have been loose anyway and would just have easily come off on a windy day. If wing tip vortices are causing tiles to dislodge I suppose you could argue that this is risk to people on the ground they might fall on.