You could simply decide not to help Astarion, and let him kill Cazador without taking his place as an ascended vampire. However, this choice will greatly harm your relationship with Astarion inBaldur's Gate 3, and he will leave your party permanently.The best option is to convince Astarion...
which deals 2d4 damage to its target, and heals Astarion for 2d4 HP. This bite gets upgraded to Ascended Bite, with 6d6 damage and healing, upon Astarion ascending into full-fledged vampire status. He also obtains Cazador's Mist ability. It's the closest players can get to ...