This video explains how Halo 2 Vista's Scripting system works.
The summary (if you don't want to watch the video):
In Halo CE, scripts were put in the data folder, and Sapien compiled the scripts. Not this time!
H2Tool compiles the scripts, and you add the scripts to your scenario via H2Guerilla (but it has to be unlocked through ) its under Scenario Resources (not the bottom one, the one on top)
If you dont understand that, watch the vid
PS: I dont mean the credit thing
