Minecart cars being weird

I made a simple cars plugin, you enter it and go in whatever direction you look. I started working on adding acceleration, but it literally seems to be ignoring the acceleration. Please take a look:

Code:

@EventHandlervoid vehicleMoveEvent(VehicleMoveEvent event)
{
   Minecart vehicle = (Minecart) event.getVehicle();
   Player player = (Player) vehicle.getPassenger();

   Vector playerVector = player.getLocation().getDirection();

   double x = Math.round(playerVector.getX());
   double z = Math.round(playerVector.getZ());

   Vector velocity = new Vector(x, 0, z);
   Vector velocityMod = new Vector(Math.max(carStartSpeed, vehicle.getDerailedVelocityMod().getX()), 0, Math.max(carStartSpeed, vehicle.getDerailedVelocityMod().getZ()));

   velocityMod = new Vector(Math.min(velocityMod.getX() + carAcceleration/20, carMaxSpeed),    velocity.getY(), Math.min(velocityMod.getZ() + carAcceleration/20, carMaxSpeed));

   if (vehicle.getFallDistance() > 0)
   {
      velocity = new Vector(x, -1, z);
      velocityMod = new Vector(1, -1, 1);
   }

   if (!vehicle.isOnGround() && vehicle.getLocation().getBlock().getType().name().toLowerCase().contains("rail"))
   {
      try {
         Method getHandle = Reflect.getOBCClass("entity.CraftEntity").getMethod("getHandle");
         Object entity = getHandle.invoke(vehicle);
         Method setPosition;
         if (Reflect.getVersion().contains("1_17") || !Reflect.NEW_NMS)
            setPosition = entity.getClass().getMethod("setPosition", Double.TYPE, Double.TYPE, Double.TYPE);
      else
         setPosition = entity.getClass().getMethod("c", Double.TYPE, Double.TYPE, Double.TYPE);
      Location position = vehicle.getLocation().clone().add(velocity);

      setPosition.invoke(entity, position.getX(), position.getY(), position.getZ());
   } catch (IllegalArgumentException | InvocationTargetException | NoSuchMethodException | SecurityException | IllegalAccessException e) {
      e.printStackTrace();
   }

   vehicle.setDerailedVelocityMod(velocityMod);
   vehicle.setVelocity(velocity);
}


@EventHandlervoid vehicleUpdateEvent(VehicleUpdateEvent event)
{
   Minecart vehicle = (Minecart) event.getVehicle();
   Player player = (Player) vehicle.getPassenger();

   Vector playerVector = player.getLocation().getDirection();

   double x = Math.round(playerVector.getX());
   double z = Math.round(playerVector.getZ());

   Vector velocity = new Vector(x, 0, z);
   Vector velocityMod = new Vector(Math.max(carStartSpeed, vehicle.getDerailedVelocityMod().getX()), 0, Math.max(carStartSpeed, vehicle.getDerailedVelocityMod().getZ()));

   velocityMod = new Vector(Math.min(velocityMod.getX() + carAcceleration/20, carMaxSpeed),    velocity.getY(), Math.min(velocityMod.getZ() + carAcceleration/20, carMaxSpeed));

   vehicle.setDerailedVelocityMod(velocityMod);
   vehicle.setVelocity(velocity);
}

This whole thing with velocity and derailedvelocitymod really confuses me.

I removed a lot of code, which although important, does not matter to this question.