#1 We're talking about version 2.1.4 stable here, right?
You originally said "... Using that, the object won't move at all ..."
In the comment you say: "... just manipulate the velocity of the object if it's already moving ..."
#2 So the object is moving and is immediately stopping when you set the velocity?
I have no idea which other forces work on your rigidbody. Is there friction and/or gravity and/or damping?
You might want to apply an impulse to the rigidbody instead (regularly in fixedprocess or once).