>Thanks, but I am still wondering if this would work when the separator is not known.

It just works. And a little correction to my previous reply, you don't need to use ToString(). ie:

' arbitrary types of items
Dim myList = {"Hello there at ", DateTime.Now(),". ", 4, " times ", 5, " is ", 4*5 }

' sep is unknown
dim sep as string

Dim Result = String.Join(sep, myList)

Console.WriteLine( Result )

Outputs:
Hello there at 5/6/2015 5:11:48 PM. 4 times 5 is 20