How to change the default "fullscreen" monitor?

I have two monitors, one attached to my Nvidia graphics card using HDMI, the other one using VGA (using a DVI-VGA adapter). Most full-screen games (that even recognizes that there are two monitors) uses the secondary monitor. The primary monitor is correctly set when I check from nvidia-settings, xrandr or KDE system settings. My current workaround is to simply disable monitor 2 when playing a game, but that's not satisfying. How can I effectively set the primary screen?