DynamicJsonDeserializer.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Ajax;
using RestSharp;
using Newtonsoft.Json;
using RestSharp.Deserializers;

namespace Helpers
{
	public class DynamicJsonDeserializer : IDeserializer
	{
		public string RootElement { get; set; }
		public string Namespace { get; set; }
		public string DateFormat { get; set; }

		#region IDeserializer implementation

		public T Deserialize<T> (IRestResponse response)
		{
			return JsonConvert.DeserializeObject<dynamic>(response.Content);
		}

		#endregion

	}
	
}

gistfile1.txt

Criar uma classe que herde de restclient e criar métodos para facilitar o uso dos verbos http e encapsular a criação do request e a definição do custom serializer

HomeController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Ajax;
using RestSharp;
using Newtonsoft.Json;
using RestSharp.Deserializers;
using System.Text.RegularExpressions;
using VideoApi;
using Helpers;

namespace Controllers
{
	public class HomeController : Controller
	{
		public ActionResult Index (string q)
		{
			if (string.IsNullOrWhiteSpace (q))
				return View ();

			var information = Regex.IsMatch(q, @"youtube\.com")? GetYoutubeDetail(q) : GetVimeoDetail(q);
			var model = new VideoViewModel {
				Title = information.title,
				Duration = information.duration,
				Thumbnail = information.thumbnail
			};

			return View (model);
		}

		private dynamic GetVimeoDetail(string url)
		{
			var code = Regex.Match (url, @"vimeo.com/\(\d+)$");
			if (!code.Success)
				throw new ArgumentException("invalid url");

			var client = new RestClient("http://vimeo.com/api/v2/video/{videoid}.output");
			client.AddHandler("application/json", new DynamicJsonDeserializer());

			var request = new RestRequest("videos/{videoid}", Method.GET);
			request.AddUrlSegment("videoid", code.Groups[1].Value);

			var model = client.Execute<dynamic>(request).Data;

			var title = model["title"];
			var thumbnail = model["thumbnail_medium"];
			var duration = model["duration"];

			return new {
				title, thumbnail, duration
			};
		}

		private dynamic GetYoutubeDetail(string url)
		{
			var code = Regex.Match (url, @"youtube.com/\watch\?v=(\S+)$");
			if (!code.Success)
				throw new ArgumentException("invalid url");

			var client = new RestClient("https://gdata.youtube.com/feeds/api");
			client.AddHandler("application/json", new DynamicJsonDeserializer());

			var request = new RestRequest("videos/{videoid}?v=2&alt=json", Method.GET);
			request.AddUrlSegment("videoid", code.Groups[1].Value);

			var model = client.Execute<dynamic>(request).Data;

			var title = model["entry"]["media$group"]["media$title"]["$t"];
			var thumbnail = model["entry"]["media$group"]["media$thumbnail"][1]["url"];
			var duration = model["entry"]["media$group"]["yt$duration"]["seconds"];

			return new {
				title, thumbnail, duration
			};
		}
	}
}

VideoViewModel.cs

using System;

namespace Models
{
	public class VideoViewModel
	{
		public string Title {
			get;
			set;
		}

		public string Duration {
			get;
			set;
		}

		public string Thumbnail {
			get;
			set;
		}
	}
}