After raining or after washing, inside my impreza 2006 rear lamps have water vapour appear on the glass which cannot be clean from the outside.
Is this normal for impreza?

I've been told by my dealer that this is normal and there is a hole behind the lamps for the water vapour to evaporate.
Is this true?

it is true that the hole is there to evap. off the remaining water but you should go back to the dealer and have them replace the lights, when you are washing the car this problem should not happen, they should be able to replace these for free.