How do I search a list for a Vector3?

I have a list that holds a number of Vector3s, and before I add another Vector3 to the list, I want to check to make sure the list doesn't already contain that value. When I use list.Exists(), it gives me an error:

"The best overloaded method match for `System.Collections.Generic.List.Exists(System.Predicate)' has some invalid arguments"

I've tried inputting the value I need to search for in different ways, but I always get this error. Does anyone know what I'm doing wrong? The error occurs at line 88 in the CheckForValidSpots function. Any advice would be super helpful!

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class LevelCreatorScript : MonoBehaviour {
     
     public GameObject roomPrefab;
     
     public int roomCount = 5;
     
     int doorCount = 0;
     
     public float roomHeight = 15.0f;
     public float roomWidth = 24.0f;
     
     List<Vector3> pastRooms = new List<Vector3>();
     Vector3 currentRoom;
     List<Vector3> futureRooms = new List<Vector3>();
     
     List<Vector3> validSpots = new List<Vector3>();
     
     List<Vector3> doorDirections = new List<Vector3>();
     
         
     void Start () {
         futureRooms.Add(new Vector3(0, 0, 0));
         CreateLevel();
     }
     
     void CreateLevel() {
         while ( (futureRooms.Count + pastRooms.Count + 1) < roomCount ) {
             
             if (doorCount == 0) {
                 
                 validSpots.Clear();
                 
                 if (true) {
                     
                     pastRooms.Add(currentRoom);
                     Debug.Log( "Adding current room: " + currentRoom + "to pastRooms at Index[" + (pastRooms.Count - 1) + "].");
                     
                 }if (futureRooms.Count > 0) {    
                     
                     currentRoom = futureRooms[0];
                     futureRooms.RemoveAt (0);
                     
                     if(pastRooms.Count == 0) {
                         doorCount = Random.Range(2,5);    
                     }else {
                         doorCount = Random.Range(1,5);
                     }
                     
                 }
         
             }else {
                 
                 //tries to make a new room
                 
                 //determine valid placement spots for new rooms
                 //puts all valid placement spots in a new list: validSpots<vector3>
                 
                 //if validSpots.count < doorCount, DoorCount = validSpots.Count
                 //randomly chooses between one of the rooms to make a new room
                 
                 
             }
             
             
             
         }
     }
     
     void AddDirections(List<Vector3> directions) {
         
         directions.Add(new Vector3(0, 0, roomHeight));
         directions.Add(new Vector3(roomWidth, 0, 0));
         directions.Add(new Vector3(0, 0, -roomHeight));
         directions.Add(new Vector3(-roomWidth, 0, 0));
         
     }
     
     void CheckForValidSpots(Vector3 roomLocation) {
         
         for (int n = 0; n < 4; n++) {
             
             Vector3 locationToCheck = new Vector3( roomLocation.x + doorDirections[n].x,
                 0, roomLocation.z + doorDirections[n].z);
         //HERE IS WHERE I GET THE ERROR
             if( !pastRooms.Exists(locationToCheck) && !futureRooms.Exists(locationToCheck) ) {
                 
                 validSpots.Add(locationToCheck);
                 
             }
         }
                     
     }
                 
 }