How do vectors work as far as projectiles are concerned in minecraft/bukkit? AFAIK projectiles have some kind of drop but the vectors have no acceleration or velocity, just a coordinate, what is this coordinate? the block it is going to hit, the block the player is looking at, something else?
Projectiles have velocity vectors, not coordinate vectors (that is what Locations are for). Projectile falls because the velocity vector y component is reduced by dy each block/tick/whatever. I hope I understood what you meant.
What is this type called? also does the server/minecraft decrease the y velocity or do the vectors have acceleration?
What do you mean by type? A vector is just something that has a direction. That something might be acceleration, electric field or anything else you want. If we are talking about entity.getVelocity(), then it's a velocity vector. As far as I know, mc doesn't have a literal acceleration vector. (Constant) vectors don't have acceleration.
civ77 If you still don't understand: the "coordinates" are the direction and speed. So 1.0D on the Y axis would move it upwards, -1.0D downwards. But beware that 1.0 may be way to fast. Just experiment with that.