You see, Vintage is a special format where (almost) all cards from Magics history can be played in some form or another
There are graveyard decks that mill their own library, but these are rarely described as "mill decks." There is also some disagreement about whether decks that mill their opponents for value, like UB Rogues, count as "true" mill decks
It also gets even better with Sazh's Chocobo and other +1/+1 counter cards
Note that if it was forked, you cannot win with it, as that won't count as "casting," but something that casts it for free from a non-hand source will count the first time, so something like Golos, Tireless Pilgrim can count as the first time you cast it, and net you seven life and stack it seven down
(7/25/2025) If a permanent entering the battlefield at the same time as Starfield Vocalist (including Starfield Vocalist itself) causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time