To a great extent, Siddhartha learns the power of detachment from Vasudeva.  The idea that Vasudeva represents the river and eternity is reflective of this.  Siddhartha is charged with learning ow to live a life of detachment, not being attached to the results of one's actions.  It is this attachment that Vasudeva teachers becomes the source of all pain.  If Siddhartha's quest to understand the true nature of being is to reveal anything in way of truth, it resides in this idea.  Consider what Vasudeva tells Siddhartha to this end:"You will learn, but not from me. The river has taught me to listen; you will learn it too."  This is where Siddhartha understands Vasudeva to be so powerful and compelling a figure.  It is for this reason that he is able to view Vasudeva as "motionless," for he has moved into a realm into which detachment is evident.  His identification of Vasudeva with "eternity" and "the river" is reflective of this.  Siddhartha's observation reveals how one perceives reality when one is detached from it, when action is to be taken, but not for its results or its attached value.  It is through this where disruption is evident.  For this reason, Siddhartha perceives Vasudeva's nature and makes his observation of Vasudeva's being as one that is both a part of this world and yet not.