Hello,
Yes indeed Vaadin is really great 
In our company we use it quite a lot for large scale and enterprise applications both on the intranet and also on the internet and we are very happy with it.
I agree that it is ideal for web applications and certainly for applications running on a safe/calm environment as the intranet, but works nicely on the internet as well. Now if you want to build a web page/site kind of thing, well i think you could do better with some other framework or raw nice technologies (i.e. jsp, jsf) or even other platform/technology i.e. php . … .unless we are talking about a portal, with portlets and stuff in which case vaadin and liferay work nicely together for building very nice systems.
Well if you publish your web app on the internet you have to take care of things that you would anyway regarding web applications, i.e. caching (at the web server and application server or even use a reverse proxy etc), small sized images, sprites, well designed html/custom templates in addition to maybe custom widgets for vaadin especially if you need something that is used extensively and you want to save trips to the server based on some logic etc
Generally you can do a lot… we have published some applications on the internet www.e-prices.gr (in a bit beta condition - this has very few optimisations regarding the web stuff and slow load times are basically because of that, very significant…however we are talking about a lot of data - millions of records and a load time in the range of 2-8secs), e-services.minagric.gr (here you can’t see much , but you see that it loads fast working nicely with jquery and everything loads as the user moves around the app etc)
We use jpa all the way, without any vaadin addon although there is one i think that might assist…It all depends on your architecture and how you integrate your technologies. In general you are very flexible.
Sorry if i’ve missed an answer…it’s running on liferay with vaadin stuff here and there as portlets as far as i can tell… and it is nice
1 - performs nicely, although you could optimise some components or make your own easily, depending on your system reqs… also take care of your layouts and your java code. Working on server and building objects takes memory and you have to handle that nicely otherwise your application server/servlet container might go slow.
2 - very cool lookin i think and easily customizable
3 - scalable yes, since you are running everything on the server you are flexible with your architecture
4 - its pure java if you want or any other web technology you like or know…so i think it’s up to the teck stack you choose
p.s. since i’ve used in the past adf a lot, trust me it’s much faster than adf (i.e. adf faces) , i’ve even compared an implentation with php app and of course the power of java shined having a beautiful vaadin ui .