Statement from Riot games on their website;
SO, WHAT HAPPENS AT THE END OF THE CLOSED BETA?
We weren’t kidding about things coming to an end. In order to start the playing field off in earnest, progress and content will be wiped when the Closed Beta comes to a close.
BUT WAIT! WHAT HAPPENS TO THE STUFF I BOUGHT?
While you won’t be able to keep any purchases from the Closed Beta, you will be refunded the monetary worth of your purchase. That means you’ll be entering the full game with your VP in hand.