I keep seeing online that he either owned a bulletproof vest but was not wearing it on the day he was assassinated, or was indeed wearing it but was shot in the neck rendering it useless.
He owned a bullet proof west made from silk invented by casimir zegelen - a polish / Austrian inventor.
He did not wear it in that day though - it is doubtful Though if this would have saved him - since as you stated correctly he was shot in the neck when he leaned over his wife who was shot first.