actually there was a bug before which we fixed now in 1.5.6. 😉
To use the behavior from before, please set the System::displaySleepEnabled property to true in your Component.onCompleted of GameWindow. Its default value is false, because in most games you do not want your game to let the device go to standby/screen darkened mode.
See this bugfix also in the changelog: http://v-play.net/doc/vplay-updates/#v-1-5-6-2014-04-02