Is the session sharing really a requirement for you? If a node goes down what you lose are the currently running sessions and sessions need to be recreated (users log-in again etc.) If you do you need the session replication, you need to ensure that everything in your session is serializable.
Most of the time clustering is needed to ensure availability for concurrent users. For that the “sticky sessions” are the way to go.This ensures that each request for a session is pointed to the same cluster node. That is load balancer and/or Web Logic specific configuration and doesn’t require anything special in Vaadin code/configuration.