Van Gogh, I found this out a bit late myself. My workaround via help from MacGuba and others was to copy and paste the elements from the intro into the mission itself and just have triggers activate it at the start of the mission.
Scripting it is more reliable , but you can get something to work this way. After the "intro" is finished, you can have a trigger to deletevehicle all unnecessary vehicles, objects, units, etc. to help ease lag in your game. Hope that helps! ;D