Adamo XPS + DisplayPort to VGA dongle limited to 640x480

My colleagues often uses his Adamo XPS to make presentations. After connecting it to a projector he is often limited to 640x480, which is useless. At one point it seemed that connecting the adapter to the projector first (then to the PC) allowed for higher resolutions (e.g. 1024x768), but that behaviour is not consistent.

What does it take to use the native resolution of a VGA projector via the dell DisplayPort to VGA adapter?