Complex sentence: Vancouver Technical School doubled as the exterior for Smallville High, since the school had the "mid-American largess" wanted by the filmmakers and was in keeping with Millar's idea that Smallville should be the epitome of "Smalltown, USA".
Split into simpler sentences: Vancouver Technical School doubled as the exterior for Smallville High, as the film makers believed Van Tech had the "mid-American largess" they wanted. This was in keeping with Millar's idea that Smallville should be the epitome of "Smalltown, USA".