Is it possible to make an operating system using VB.NET?
Yes, it is possible.
But first you will have to identify what an operating system is; and then define what would you want in your operating system? An operating system does a lot of work on the background as well as on the foreground; there are applications, memory, threading, network, ports, a separate world that make things work.
I would agree with Aviad that probably your OS may be called a .NET CLR, but thats what that would be called. An answer to your question is that it is possible provided that you define your OS and probably limit yourself to what you really require.