There is no direct way to animate a vector input in the node editor. For example, if you add a "displace node" in the compositor, there is a vector input that specifies the displacement direction. If you click on the vector input, it brings a drop down with three inputs for x, y, and z. Currently, blender will not let you add a keyframe for these inputs.

I don't know if allowing these inputs to be animated would be the best design choice though. It might be better to have a "Vector" input node, that takes three inputs that can be animated. I guess you can use the "Combine RGB/RGBA" nodes as a workaround; but, maybe it would be nice to have an explicit "Vector" input node.