The way I have this working is for a parent view that calls a surface controller to render a child partial (and then subsequently returning a partial from an ajax post).

Let's say I have the following controller.

using System.Web.Mvc;
using MyWebsite.ViewModels;
using Umbraco.Web.Mvc;

namespace MyWebsite
{
    public class ThingController : SurfaceController
    {
        [ChildActionOnly]
        [HttpGet]
        public ActionResult Item(int id)
        {
            ItemViewModel viewModel = new ItemViewModel();
            return PartialView(viewModel);
        }
    }
}

This renders the ~/Views/Partials/Item.cshtml partial

The Item.cshtml partial is declared like this:

@using MyWebsite.ViewModels;
@model ItemViewModel

@*HTML*@

I initially render it from a parent view like this:

@Html.Action("Item", "ThingController", new { eventId = Model.Content.Id })