rotate a vector3 ?

i want to rotate a vector3 in a downwards angle of 35 degree

say i have a which points north west roughly which is about vector3(0.77, 0 0.77);
all the vector3 i use or directions have a 0 in Y , so that they are flat

i want to tilt it down wards by 35 degrees, so still facing the same direction but also downwards
what code would i use ?

You can read about rotations here:
Unity Script Reference: Transform.Rotate()
Unity Gems: Quaternions and rotations(). But I suppose this would work:

#pragma strict

var lookAtTarget : GameObject;
var upDownAngle : float = 35;

function Update () {
	transform.LookAt( lookAtTarget.transform );
	transform.rotation.eulerAngles += new Vector3( upDownAngle, 0f, 0f );