I feel like an oversight or a missed opportunity regarding Astarion was that he couldn’t perform Vampire Bite on Cazador during the Ascension ritual fight. Like I understand that you can’t use it on undead but with the trouble that the game established with Astarion explaining how a vampire spawn can turn into a true vampire, you would think that biting Cazador would be an exception. I think it would’ve been a great and ironic twist for Astarion to be able to become his own master that way as well. But oh well, I’m guessing it would’ve lessen the impact of certain outcomes to Astarion’s questline.
It was never stated in the game that the true vampire needs to give permission/consent, just that they usually would never give them the opportunity to drink their blood. In theory, a spawn could force their way if given the chance.
It is, Astarion explains it pretty well, he says that a spawn is not able to bite the master unless the master allows it, and that basically never happens because the master would lose a slave in favor of a competitor.
Unless of course something was wrong with the spawn, like maybe if it had a tadpole in its eye or something.