That should work without any problems. In my HomeController Index action I add a message to the ViewBag:
ViewBag.Message = "Welcome to ASP.NET MVC!";
On the Index View I add the partial view:
@Html.Partial("ViewName")
And on the partial view I render the message:
@ViewBag.Message
From the comments below: there seems to be a problem when you pass a model to the partial view. Then you can refer to the original ViewBag with
@ViewContext.Controller.ViewBag.Message