Whenever a boss is addressed they always say Don Corleone, Don Barzini, etc. However, isn't Don a title that always goes with the first name and not the surname? Vito should be addressed as Don Vito and not Don Corleone, same with all other dons.

You're technically right but its Hollywood Don Kostic.

A few people (the slumlord in II) call him "Don Vito" but yeah mostly he is referred to as Don Corleone.