Azure Vault key access sample using applicationid and secret. It is slightly modified MS sample

program.cs

using Microsoft.Azure.KeyVault;
using Microsoft.Azure.KeyVault.Models;
using Microsoft.IdentityModel.Clients.ActiveDirectory;
using Microsoft.Rest.Serialization;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace azureKeyV
{
	class Program
	{
		static void Main(string[] args)
		{
			var clientId = "GUID value";
			var secret = "secret (Base64 encoded string)";

			var clientCred = new ClientCredential(clientId, secret);
			var keyVaultClient = new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(
				   (authority, resource, scope) => GetAccessToken(authority, resource, scope, clientCred)));

			var vaultAddress = "https://something.vault.azure.net";
			var keyName = "keyName here";

//enumerates all secrets in the vault
			var secrets = keyVaultClient.GetSecretsAsync(vaultAddress).ConfigureAwait(false).GetAwaiter().GetResult();
			foreach (var s in secrets)
				Console.WriteLine("secret {0}", s.Identifier);

//enumerates all keys in the vault			
      var keys = keyVaultClient.GetKeysAsync(vaultAddress).ConfigureAwait(false).GetAwaiter().GetResult();
			foreach (var k in keys)
				Console.WriteLine("key {0}", k.Identifier);

			var retrievedKey = Task.Run(() => keyVaultClient.GetKeyAsync(vaultAddress, keyName)).ConfigureAwait(false).GetAwaiter().GetResult();

			PrintoutKey(retrievedKey);

			Console.WriteLine(Convert.ToBase64String(retrievedKey.Key.N));

			Console.ReadKey();
		}
		
		/// <summary>
		/// Gets the access token
		/// </summary>
		/// <param name="authority"> Authority </param>
		/// <param name="resource"> Resource </param>
		/// <param name="scope"> scope </param>
		/// <returns> token </returns>
		public static async Task<string> GetAccessToken(string authority, string resource, string scope, ClientCredential cred)
		{
			var context = new AuthenticationContext(authority, TokenCache.DefaultShared);
			var result = await context.AcquireTokenAsync(resource, cred).ConfigureAwait(false);

			return result.AccessToken;
		}

		private static void PrintoutKey(SecretBundle value)
		{
			Console.Out.WriteLine("Key: \n\tKey ID: {0}\n\tKey type: {1}",
				value.SecretIdentifier, value.Kid);

		}

		private static void PrintoutKey(KeyBundle keyBundle)
		{
			Console.Out.WriteLine("Key: \n\tKey ID: {0}\n\tKey type: {1}",
				keyBundle.Key.Kid, keyBundle.Key.Kty);

			var expiryDateStr = keyBundle.Attributes.Expires.HasValue
				? keyBundle.Attributes.Expires.ToString()
				: "Never";

			var notBeforeStr = keyBundle.Attributes.NotBefore.HasValue
				? keyBundle.Attributes.NotBefore.ToString()
				: UnixTimeJsonConverter.EpochDate.ToString();

			Console.Out.WriteLine("Key attributes: \n\tIs the key enabled: {0}\n\tExpiry date: {1}\n\tEnable date: {2}",
				keyBundle.Attributes.Enabled, expiryDateStr, notBeforeStr);
		}
	}
}