Regarding the claim that the lore of vampire brides is supposedly outdated because it’s based on Van Richten's Guide to Vampires, published in 1991, and that this lore isn’t mentioned in the current edition of DnD 5e, specifically in the 2021 edition of Van Richten's Guide to Ravenloft:
First of all, vampire brides are also mentioned in Van Richten’s Monster Hunter’s Compendium from 1999.
But most importantly, the 2021 edition of Van Richten's Guide to Ravenloft directly references Van Richten's Guide to Vampires.
So…
If we disregard the lore of Ravenloft (even though the game clearly alludes to it) and focus solely on the game itself:
Player: How does someone become a vampire, exactly? Astarion: It's simple. Just find a vampire that will drink your blood and turn you into a vampire spawn: their obedient puppet. In theory, the next step is to drink their blood. Once you've done that, you're free and a true vampire. Player: 'In theory'? Astarion: People think the biggest threat to a vampire is a cleric with a stake. It's not. The biggest threat to a vampire is another vampire. They're scheming, paranoid, power-hungry beasts. So why would any vampire give up control over a spawn to create a competitor?
Player: So they bite you, you bite them? Astarion: Yes and no. The problem is once you're a vampire spawn, they completely control you. They have to allow you to bite them. And why would they do that? Vampires are power-hungry creatures. They won't lose a servant to create a competitor. Trust me. It doesn't happen.
Once AA gives Tav his blood, they can no longer be considered a spawn—according to the lore, they become a true vampire. This is confirmed both by in-game dialogues and the D&D 5E lore on which the game is based.
Astarion: Nearly two hundred years and I never came back. Not since the night I woke up down there. I had to punch a hole in the coffin and claw my way through six feet of dirt. Then when I finally broke the surface, retching up dirt and congealed blood, Cazador was waiting. From that day on I was his. Until today. Moreover, we never see AA mentally controlling Tav the way Cazador controlled his spawns—which would be undeniable proof of their status as a spawn. The game only presents a dialogue about freedom, which does not contradict the concept of Tav as a bride but also does not confirm any magical control that could prove otherwise.
So if you believe that the Bride Theory is unfounded and that the process of becoming a vampire bride doesn’t align with what is shown in the game, then consider this: it has even less in common with the process of becoming a spawn. The turning into a spawn is described in detail in the game, and clearly not without reason—and Tav's/Dark Urge's turning is not just different in some aspects, it has absolutely no similarities with it.
No matter which lore you take, Tav is not a spawn.
Maybe I should make a post with counterarguments to all the most common arguments against the Bride Theory.