If you just need the object from an enumerable, at a given index, here is a simple function that does it:

    public static object GetObjectAt(IEnumerable enumerable, int index)
    {
        int i = 0;
        foreach (object obj in enumerable)
        {
            if (i == index)
                return obj;

            i++;
        }
        throw new IndexOutOfRangeException();
    }

And in your case, you would do this:

        object oo = GetObjectAt((IEnumerable)(WeakReference)vals[2], (int)vals[4]);

Of course there are alternatives that look more sexy (see other answers), with fancy linq queries and cool C# 4 dynamic new trendy stuff :-) but, in general, if you don't "need" the T type (in your example code, you don't need it), you don't need generics. This solution is actually supported with any .NET Framework and C# versions, from 1 to 4.