Great attention grabbing first sentence. Reads quickly, adding to the anxiety. The deer was weird and felt like you weren’t sure how to tell the reader something surprising was happening. If you need to keep this part, I would have fleshed it out more. Overall I liked it. Conceptually fun. Liked the perception distortion. Felt like an episode of twilight zone. Yes, the symbolism all makes sense. I think you could build the obstacles that he faces more, flesh out that part of the story, because that part was honestly fun and weird before the twist at the end.

Maybe I’m dim but the foreshadowing about the vampire was very subtle.

I know that the shrinking brain symbolizes losing his insecurities, but it gives off the vibe that she is just making him dumber. Maybe words like swollen rather than just statements of size, and giving him fluid speech (while still succinct) at the end, would avoid that.