I can see using Spring if you already have it, but for the new project, what's the point? I would go directly with Java EE 6 (ejb3, jsf2.0, etc.)
If the client is fine with Flex, go for it. Use BlazeDS or similar - no mvc. You might spend more time on that part (exchanging data between server and client) but you have full control on both sides.
Don't use Vaadin, unless you want to kill your browser. Plus, you spend more time on getting around the code once your pages become more complex. Also, your mindset will need to be completely changed and anything you know about standard front end development will be waste. The argument that you don't have to use HTML or JS doesn't make much sense. You still have to know it even if you don't use it. It renders to HTML and JS eventually. Then try to debug it - make sure you got few days for simple stuff. Plus, I cannot imagine web developer who doesn't know html/js.
I just don't understand why people are trying all those abstractions instead of using Java EE directly.
