Camera rotation with mouse.

Hi. I'm trying to make that camera can rotate around object with the mouse,Problem in the last line of code,this line was taken from "Scripting API" "transform.RotateAround(Vector3.zero, Vector3.up, 20 * Time.deltaTime)"and its working fine as it is. But when i'm trying to change Vector3.up even on "Vector3 (0,1,0)"I have an error.

 using UnityEngine;
 using System.Collections;
 
 public class camera_switch : MonoBehaviour {
 
     public float ScreenHi = 0.12f;
     public float ScreenWi = 0.12f;
     public float MenuPosX = 5f;
     public float MenuPosY = 20f;
     public Transform Obj1;
     public Transform Obj2;
     public float CamSpeed = 1.0F;
     private float time;
     private Vector3 currentPos;
     private Vector3 toPos;
     public float mouseSens = 2.0F;
 
 
     
         void OnGUI() {
         GUI.Box (new Rect (MenuPosX, MenuPosY, ScreenWi, ScreenHi), "Switch Menu");
     
         if (GUI.Button (new Rect (MenuPosX + 10, MenuPosY + 20, ScreenWi - 20, ScreenHi - 70), "Obj 1")) {
             currentPos = Obj1.position;
             toPos = Obj2.position;
             time=0;
             
 
         }
         if (GUI.Button (new Rect (MenuPosX + 10, MenuPosY + 60, ScreenWi - 20, ScreenHi - 70), "Obj 2")) {
             currentPos = Obj2.position;
             toPos = Obj1.position;
             time=0;
 
 
         }
     }
 
 
         void LateUpdate() {
 
         float mouseX = Input.GetAxis ("Mouse X") * mouseSens;
         float mouseY = Input.GetAxis ("Mouse Y") * mouseSens;
 
 
             time += Time.deltaTime * CamSpeed;
 
         transform.LookAt(Vector3.Lerp(currentPos,toPos,time));
 
         transform.RotateAround (toPos, Vector3 (mouseX, mouseY, 0) , 20 * Time.deltaTime);    
 
         }
 
     }