function Start () {
    // Shader vectors are always Vector4s.
    // But the value here is converted to a Vector3.
    var value : Vector3 = renderer.material.GetVector("_SomeVariable");
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Start() {
        Vector3 value = renderer.material.GetVector("_SomeVariable");
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Start():
        value as Vector3 = renderer.material.GetVector('_SomeVariable')