Hi Francesco,
I am afraid there is nothing that can easily be integrated in VA ST. Both the Composition Editor and WindowBuilder have spent a good decade in some kind of winter's sleep. This is really sad and frustrating, especially in the light of the fact that Instantiations has had a long tradition in building excellent GUI building tools, the last of which having been WindowBuilder (GWT, SWT and Swing Designer) for Java/Eclipse.
So if you really need to build modern-looking, fancy GUIs, Smalltalk in general is not a good area for you - not only VA Smalltalk. The best thing I can think of is some level of integration with external processes that drive a GUI and keep your business code in VAST. If you use DLL-bindings or OLE or some other technology, this will always be hard work.
On the other hand, Instantiations has taken over VAST from IBM in 2005, and after a few years of very low investment, there has been a lot going on under the covers in recent years. VAST as an IDE has made huge leaps forward with much better editing, code competion, syntax highlighting and such. If you compare VAST 8.6.x to 8.0, things feel so much different.
The current focus lies in a new 64-bit VM, and before that a lot of work went into making Seaside work on VAST for web applications. I can tell you this works quite well, because we've been using Seaside on VAST in several projects.
So here is hoping that Instantiations comes back to their roots and picks up on the GUI front once the dust settles on the upcoming 64-bit VM. I personally think WindowBuilder is the best possible base for a future GUI building tool in VAST, but it is clear that even if Instantiations decides to put their priorities towards GUI building tools (which I personally think they should), we will not see anything reasonably improved before 2018. I know this sounds depressing (which I think it is), but realistic.
And then, there is this important question that every GUI builder must answer: what kind of GUI look&feel is the preferred one? What libraries to use? Look at MS Office, which has been a reference point for many years: every two years it looks completely different. As a GUI developer, you are constantly running after the current GUI-style-du-jour. Remember: VAST runs on Windows, AIX and Linux. So libraries like GTK seem to be a good fit, because there is a GTK for Windows. BUT this still is not "native Windows" and will always be a least common denominator between modern Linux GUIs and WIndows. OTOH, GTK opens new options like Mac OS.
So this whole thing is complicated and I am glad I am not in a position to make a decision here. It's clear something has to be done about VAST's GUI tools. But there is no obvious choice for a new base...
So my short answer is: WindowBuilder is currently the best thing you can get.
Joachim