I resisted installing gumbo until it reached public beta, and having just done so started up a project only to find that using the auto complete VBox amongst others seemed to have disappeared. Although using the VBox tag worked in exactly the same way as previously.
This made me a tad curious, was this a bug with the beta, I doubted that as that would have been a major issue. So i figured it must have been for a reason, and it was. A very good reason it turned out!
Due to the large number of new component classes and the different namespaces, Flash Builder will now only show the recommended components when using auto complete (unless you press Ctrl + Space, then you cycle through various options such as ‘show all’). This I like. Great idea and a great way to get those folk set in their ways to find out more about the new classes.
So what is recommended in place of the old VBox? This took a little while of searching Google, but I’ve found this article in the Adobe Dev Connection, http://www.adobe.com/devnet/flex/articles/flex3and4_differences_04.html. This lists the old versus new and shows that the VBox is now replaced by the VGroup.
So there you go, should you not find the class in the auto complete help to start with, press Ctrl + Space then check out the above link to find out what you should really be using.