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Vaadin does not persist the session; Vaadin *uses* the HTTP Session, as provided to it by the application server (e.g. Tomcat).
If you wish to prevent the session being serialized to disk, you need to configure the application server appropriately. I've never had to do this, but I believe if you search this forum, Joonas has posted how to configure Tomcat not to persist sessions. Other app servers will need different configuration - I'm afraid it's a case of reading their manuals.