Iran’s right to self-defense under article 51 of the UN charter takes precedence over the Vienna convention on diplomatic relations. When the US took the Shah in under the pretense of medical treatment, it was entirely plausible that they would resort to the same plan they used in 1953 during the CIA sponsored coup d’état that overthrew the popular government of Mossadegh. So the Iranians were acting in self-defense when they stormed the embassy.