Manipulating a vertex on a plane

How can I go about getting/translating a vertex of of a plane/cube?

you need to access the vertices through the Mesh function this way

function Update () {
    var mesh : Mesh = GetComponent(MeshFilter).mesh;
    var vertices : Vector3[] = mesh.vertices;
    var normals : Vector3[] = mesh.normals;
    for (var i = 0; i < vertices.Length; i++)
        vertices_.transform.position = vertices*.transform.position + Vector3(0,(Random.Range(-1,1),0)*_

mesh.vertices = vertices;
}
when you have all your vertices into an array, you can manipulate them as you want.
beware, you may not know :
that there may be several vertices sharing the same position
and their position is also stored in LocalSpace (that means their coordinates are relative to the object position)
Unity Documentation about the Mesh Class is [here][1]
_*[1]: http://docs.unity3d.com/Documentation/ScriptReference/Mesh.html*_