In the middle of World War I, a sullen 17-year-old boy accidentally wanders into the world of a glimmering vampire of the same age; within their journey to discover their life meanings they run into similar problems along the way and eventually need each other to get out of their problems.

Hello bardyshiro, I believe your logline needs a little work.

1: “In the middle of World War I,..”

You tell us the story takes place in the middle of WW1 but then nothing in the rest of the logline seems to have anything to do with the war. If the war isn’t important to the story I would leave it out of the logine.

2: “A sullen 17 year old boy accidentally wanders into the world of a glimmering vampire”

Is this an actually world, like Narnia, or is this just your way of telling us the lead meets a vampire?

3: “they run into similar problems along the way and eventually need each other to get out of their problems.”

In all stories the characters run into problems then get out of problems. It would help if you would tell us specifically what the main problem is.

I believe the main problem with this logline is, the lead doesn’t have a goal and as such there is nothing standing in the way of that goal. As a result the logline comes across as a little vague.

Add a goal and something standing in the way and it will tighten up your logline.

Hope that helped, good luck with this!