In some cases, Vectorworks will export a custom object created with a mix of 3D modelling tools the wrong way. This sometimes included mirrored appearance and lighting in Lumion.
In these cases, LiveSync for Lumion has a 'Repair mode' button. That needs to be ticked when exporting. If the issue occurs which the 'Repair mode' button is ticked, then it needs to be unticked. Unfortunately, this is the VW paradox.
It is highly recommended that objects with this behaviour be exported separately from the rest of the model due to the fact that some would work with 'Repair mode' and the others without.
Note: As it's potentially a bug but external its worth logging rather than waiting for a fix by keeping only in Tickets.