Google AppEngine _vac and _ah_session data limit and cleanup

I deployed a small app to GAE recently, and a couple of days ago i start getting server errors and noticed that the datastore usage is nearing the limit for the free version ( 1GB ) while i’m not using anywhere near that much for my own data. After some digging, it turns out that the _vac and _ah_session data stores took up most of the 1GB.

As far as i understand it, these are vaadin’s session data, around 80KB-120KB in size for each “record”. I was expecting to see expired data to be deleted automatically, but it seems that this is not the case. Am I doing something wrong or are we supposed to do some manual cleanup with cron jobs or some other method?

Thanks in advance.

GAEApplicationServlet javadoc

Thank you.
Apologies if this is more a GAE issue rather than a Vaadin issue.