Yeah that wording is a little wonky and it's missing the needed (!) It's A Bite attack.
1) ... Grappled by You , or Incapacitated , or Restrained
2) ... melee attack ; presumed to be the Bite otherwise things get weird
3) ... if hit
4a) ... then special damage happens with the hp max reduced
4b) ... Feast of Blood ability triggers from the hit
5opt) ... if hp max (NOT hp damage) reaches zero, target is dead
Pretty much has to be a grapple build to trigger Feast,
but then you get up and waltz around the battle field,
wait til dude drops , come back around , grapple ,
ready Bite for 1 hp heal from somebody else = dead dead.