If you want to make vampires immune to disease, the checks for whether a character can contract a disease are scripted in common/scripted_triggers/20_health_triggers.txt. The existing trigger checks for the "immune_to_disease" character flag, so the easy approach will be to set that flag on all vampire characters instead of trying to override the trigger with a customized version.

The code would look like
in any code path that adds the Vampyr trait to a character. You'll probably also need to run an effect at game start (i.e. in the on_game_start_after_lobby action) to look for characters that had the Vampyr trait added in the ruler designer.