Soma/Sein is also 18, almost 19, as of his canon point! He's planning on college/Eddan equivalent but I headcanon his dad might've had some health issues and Sein is taking a break after graduation to help with the cafe. Maybe their cafes are far enough apart that Sein and Jaune can be friends and not rivals. XD And the idea that they "know" each other in their AU forms is kinda cool!

And yes, please befriend all the vampires. It makes things easier when Soma eventually gets the urge to nibble on someone I mean what