It could lead to a whole new set of adventures and challenges for Dawn. Maybe she'd have to learn to control her vampire urges while still trying to maintain her relationships with friends and family.
The Scooby Gang would also be affected. Xander might be more hesitant and scared, while Willow could try to use magic to help Dawn. Giles would probably be the voice of reason, trying to figure out the best course of action. And Spike, being a vampire himself, could have a complex reaction. He might see Dawn as a new addition to the vampire world or feel some sort of loyalty to Buffy and try to help Dawn resist her new nature.
One more theme is the power struggle. Vampires in these fanfics usually have certain powers that can either be a threat or an advantage in the Pokemon world. So, there could be a power struggle between vampires, Pokemon trainers, and the Pokemon themselves. This can create intense battles and complex relationships in the fanfic, as different characters try to assert their dominance or protect their own kind.
One common theme is identity. Since Dawn becomes a vampire, she has to re - evaluate who she is. Another theme is family. Buffy's reaction to Dawn's new state and how they interact as sisters in this new situation is often explored.
One common theme is identity. Dawn has to figure out who she is as a vampire, different from her human self. Another theme is family. Buffy and the rest of the gang have to deal with Dawn in this new state, which tests their family bonds.
One common theme is the struggle between good and evil. Since it involves vampires, there's often a battle between the forces of darkness (the vampires) and the forces of good, with Dawn sometimes caught in the middle.
I think there are several such fanfictions out there. You might find one on popular fanfiction websites like Archive of Our Own or FanFiction.net. But specific titles could vary.
It could change the whole dynamic. Buffy would have a personal stake in dealing with vampires in a different way now. Instead of just slaying them all, she might try to find a cure for Dawn. This could lead to her making deals with other vampires or searching for ancient magic.
One possible plot could be Dawn's struggle to control her new vampire instincts while still maintaining her relationships with the Scooby Gang. She might have to hide her transformation from Buffy at first, leading to a lot of internal conflict.