I've had this happen twice. Never was able to track it down. Its almost an intermittent thing for me. Both times, parked in the same place, differing amounts of rain, I just cant find the leak. Its rare, but out of nowhere.
I have seen a number of people saying that if they park their VX on an angle with the nose down, they see the issue. Mine is happening just from driving in the rain. It is garaged the rest of the time. Was yours at a downward angle?