What other vector do you need? From the javadoc:
/**
* <code>lookAt</code> is a convienence method for auto-setting the frame
* based on a world position the user desires the camera to look at. It
* repoints the camera towards the given position using the difference
* between the position and the current camera location as a direction
* vector and the worldUpVector to compute up and left camera vectors.
*
* @param pos
* where to look at in terms of world coordinates
* @param worldUpVector
* a normalized vector indicating the up direction of the world.
* (typically {0, 1, 0} in jME.)
*/