Vaadin + Liferay = nice and shiny UI
Vaadin + JRebel = fast development (at least according to ZeroTurnaround’s representatives)
JRebel + Liferay = fast development
Liferay + JRebel + Vaadin = … ?
I tried that a couple of months ago. It worked fine, but you need to remember to turn off automatic class reloading by Liferay, so that JRebel can do its job. I believe JRebel plugin for Eclipse does this automatically for you.