Ehm... right... how to say it? You aren't a vampire spawn, she wants to be bitten by a vampire, your Durge might be murderous, bot they aren't a vampire.
If you use a mod for that, it is still not canon.
My DUrge is a vampire spawn though. No mods involved.
Astarion turns you into his spawn if he ascends and you continue to romance him. You even get a bite attack for the happy buff.
Even as a DM, I would reject the idea as unless you are vampire/Dhampir, etc. It doesn’t count.
They are!
Oh that thing. No offence, that’s just sequence breaking I am afraid. The deal is in Act 2, you see her again in Act 3, but that’s not something the devs should include or plan for or make it make sense.
Case in point, you know Revivify? The thing that should help save us NPCs lives? Maybe change the outcomes? It doesn't work this way. Even, if we assume you get turned into Vampire Spawn by Astarion, it happens too late in the game to have any effect. The opportinity has passed.
There are certain limitations and while I can see your point, it's a fringe case at the very best. Just not something that was planned for and I think it should simply be available during Act 2. After that, she could have found another vampire spawn or something.
