We have a partner that successfully load tested an application built with the Vaadin framework. The POST requests have a plain/text content type and the responses are application/json. So, nothing that NeoLoad can't handle. From that test, no specific NeoLoad module was needed.
Update: there's an advanced action available in the Neotys labs in order to test Vaadin applications. Here are the links to the Lab to download the Frameworks and Encode/Decode files and another link to the specific documentation here.