How can I rotate(per radians or degrees) a vector around it's orthogonal axis (specified by another vector)?
I need it for a camera with the ability to roll it's up vector around it's direction axis.
I have read the examples, but they all focus on 2D-camera-rotating with a fixed up-axis.
Ok, got it 
private Quaternion _roll = new Quaternion();
private Vector3f _upAxis = new Vector3f();
private Vector3f _tempAxis = new Vector3f();
public void roll(float value)
{
_roll.fromAngleAxis(FastMath.DEG_TO_RAD * value, getCamera().getDirection());
_roll.toRotationMatrix().mult(_upAxis, _tempAxis);
_upAxis.set(_tempAxis);
}